JRC62663

99
Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Seweryn Kokot, Armelle Anthoine, Paolo Negro, George Solomos PUBSY JRC 62663 - 2010

Transcript of JRC62663

Page 1: JRC62663

Static and dynamic analysis of

a reinforced concrete flat slab framebuilding for progressive collapse

Seweryn Kokot Armelle AnthoinePaolo Negro George Solomos

PUBSY JRC 62663 - 2010

The mission of the JRC-IPSC is to provide research results and to support EU policy-makers in their effort towards global security and towards protection of European citizens from accidents deliberate attacks fraud and illegal actions against EU policies European Commission Joint Research Centre Institute for the Protection and Security of the Citizen Contact information Address Seweryn Kokot TP 480 Joint Research Centre I-21027 Ispra ITALY E-mail sewerynkokotjrceceuropaeu Tel +390332-786779 Fax +390332-789049 httpipscjrceceuropaeu httpwwwjrceceuropaeu Legal Notice Neither the European Commission nor any person acting on behalf of the Commission is responsible for the use which might be made of this publication

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A great deal of additional information on the European Union is available on the Internet It can be accessed through the Europa server httpeuropaeu JRC 62663 Luxembourg Publications Office of the European Union copy European Union 2010 Reproduction is authorised provided the source is acknowledged Printed in Italy

Contents

1 Introduction 3

2 Description of the structure 5

21 Materials 5

22 Resistance of the frame elements 8

23 Summary of the previous analyses and experiment for progressive collapse 12

3 Finite element model in SAP 2000 14

4 Linear static analysis 19

41 Before demolition 19

42 One central column removed 24

43 One left corner column removed 29

44 One right corner column removed 34

5 Linear dynamic analysis 39

51 One central column removed 40

52 One left corner column removed 47

53 One right corner column removed 54

6 Nonlinear dynamic analysis 61

61 One central column removed 69

62 One left corner column removed 73

63 One right corner column removed 77

7 Two central columns removed 78

8 Conclusions 79

References 82

List of Figures 83

List of Tables 88

A Photos from experimental destroying of columns 90

1 Introduction

Progressive collapse of structures occurs when a local failure triggers successivefailures and leads to the total collapse or a collapse disproportionate to the originalcause There were a few world-wide known examples of progressive collapses suchas the partial collapse of the Ronan Point residential apartment building (London1968) the major collapse of the Alfred P Murrah Federal Building (Oklahoma 1995)etc The first progressive collapse regulatory documents followed the Ronan Pointpartial collapse and were included into the British standards In turn after the totalcollapse of the World Trade Center towers many research activities lead to moredetailed guidelines on designing and preventing progressive collapses (eg [5] [2][8])

There are basically two approaches when dealing with the evaluation andprevention of progressive collapses in a given structure The first indirect approachconsists in ensuring that the structure satisfies prescriptive design rules (such asrequirements on structural integrity and ductility or the presence of vertical andhorizontal ties) The second direct approach uses two possibilities depending onwhether local failure is allowed or not If local failure is allowed then the structuremust be verified using the alternate load path method in which a load-bearingelement is removed from the structure If no local failure is allowed then keyelements must be designed to sustain a notional accidental action More detailedinformation on the state-of-the-art in the field of progressive collapse can be foundin the JRC Scientific and Technical Report [6]

A few years ago at the ELSA laboratory a reinforced concrete flat-slab framebuilding was tested to evaluate its safety against collapse (see [4]) First staticlinear and nonlinear analyses of the building under column removals were performedand then several columns of the building were demolished one after the otherto observe the building behaviour This experiment has shown not only that thestructure survived the demolition of two central columns but also how challengingthe structural testing against progressive collapse is Even though the columns weredemolished rather slowly using a concrete crunching machine still safety provisionsprevented the planned sequence of column removal from being followed

However buildings can be exposed to fast dynamic abnormal events suchas bomb explosions or impacts so the dynamic nature of the loading must beconsidered Therefore the purpose of this report is to re-evaluate the previouslymentioned frame building using linear and nonlinear dynamic analyses accordingto the alternate load path method In other words this study tries to answer thequestion what would have happened if the columns had been destroyed dynamically

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC-IPSC is to provide research results and to support EU policy-makers in their effort towards global security and towards protection of European citizens from accidents deliberate attacks fraud and illegal actions against EU policies European Commission Joint Research Centre Institute for the Protection and Security of the Citizen Contact information Address Seweryn Kokot TP 480 Joint Research Centre I-21027 Ispra ITALY E-mail sewerynkokotjrceceuropaeu Tel +390332-786779 Fax +390332-789049 httpipscjrceceuropaeu httpwwwjrceceuropaeu Legal Notice Neither the European Commission nor any person acting on behalf of the Commission is responsible for the use which might be made of this publication

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Contents

1 Introduction 3

2 Description of the structure 5

21 Materials 5

22 Resistance of the frame elements 8

23 Summary of the previous analyses and experiment for progressive collapse 12

3 Finite element model in SAP 2000 14

4 Linear static analysis 19

41 Before demolition 19

42 One central column removed 24

43 One left corner column removed 29

44 One right corner column removed 34

5 Linear dynamic analysis 39

51 One central column removed 40

52 One left corner column removed 47

53 One right corner column removed 54

6 Nonlinear dynamic analysis 61

61 One central column removed 69

62 One left corner column removed 73

63 One right corner column removed 77

7 Two central columns removed 78

8 Conclusions 79

References 82

List of Figures 83

List of Tables 88

A Photos from experimental destroying of columns 90

1 Introduction

Progressive collapse of structures occurs when a local failure triggers successivefailures and leads to the total collapse or a collapse disproportionate to the originalcause There were a few world-wide known examples of progressive collapses suchas the partial collapse of the Ronan Point residential apartment building (London1968) the major collapse of the Alfred P Murrah Federal Building (Oklahoma 1995)etc The first progressive collapse regulatory documents followed the Ronan Pointpartial collapse and were included into the British standards In turn after the totalcollapse of the World Trade Center towers many research activities lead to moredetailed guidelines on designing and preventing progressive collapses (eg [5] [2][8])

There are basically two approaches when dealing with the evaluation andprevention of progressive collapses in a given structure The first indirect approachconsists in ensuring that the structure satisfies prescriptive design rules (such asrequirements on structural integrity and ductility or the presence of vertical andhorizontal ties) The second direct approach uses two possibilities depending onwhether local failure is allowed or not If local failure is allowed then the structuremust be verified using the alternate load path method in which a load-bearingelement is removed from the structure If no local failure is allowed then keyelements must be designed to sustain a notional accidental action More detailedinformation on the state-of-the-art in the field of progressive collapse can be foundin the JRC Scientific and Technical Report [6]

A few years ago at the ELSA laboratory a reinforced concrete flat-slab framebuilding was tested to evaluate its safety against collapse (see [4]) First staticlinear and nonlinear analyses of the building under column removals were performedand then several columns of the building were demolished one after the otherto observe the building behaviour This experiment has shown not only that thestructure survived the demolition of two central columns but also how challengingthe structural testing against progressive collapse is Even though the columns weredemolished rather slowly using a concrete crunching machine still safety provisionsprevented the planned sequence of column removal from being followed

However buildings can be exposed to fast dynamic abnormal events suchas bomb explosions or impacts so the dynamic nature of the loading must beconsidered Therefore the purpose of this report is to re-evaluate the previouslymentioned frame building using linear and nonlinear dynamic analyses accordingto the alternate load path method In other words this study tries to answer thequestion what would have happened if the columns had been destroyed dynamically

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 3: JRC62663

Contents

1 Introduction 3

2 Description of the structure 5

21 Materials 5

22 Resistance of the frame elements 8

23 Summary of the previous analyses and experiment for progressive collapse 12

3 Finite element model in SAP 2000 14

4 Linear static analysis 19

41 Before demolition 19

42 One central column removed 24

43 One left corner column removed 29

44 One right corner column removed 34

5 Linear dynamic analysis 39

51 One central column removed 40

52 One left corner column removed 47

53 One right corner column removed 54

6 Nonlinear dynamic analysis 61

61 One central column removed 69

62 One left corner column removed 73

63 One right corner column removed 77

7 Two central columns removed 78

8 Conclusions 79

References 82

List of Figures 83

List of Tables 88

A Photos from experimental destroying of columns 90

1 Introduction

Progressive collapse of structures occurs when a local failure triggers successivefailures and leads to the total collapse or a collapse disproportionate to the originalcause There were a few world-wide known examples of progressive collapses suchas the partial collapse of the Ronan Point residential apartment building (London1968) the major collapse of the Alfred P Murrah Federal Building (Oklahoma 1995)etc The first progressive collapse regulatory documents followed the Ronan Pointpartial collapse and were included into the British standards In turn after the totalcollapse of the World Trade Center towers many research activities lead to moredetailed guidelines on designing and preventing progressive collapses (eg [5] [2][8])

There are basically two approaches when dealing with the evaluation andprevention of progressive collapses in a given structure The first indirect approachconsists in ensuring that the structure satisfies prescriptive design rules (such asrequirements on structural integrity and ductility or the presence of vertical andhorizontal ties) The second direct approach uses two possibilities depending onwhether local failure is allowed or not If local failure is allowed then the structuremust be verified using the alternate load path method in which a load-bearingelement is removed from the structure If no local failure is allowed then keyelements must be designed to sustain a notional accidental action More detailedinformation on the state-of-the-art in the field of progressive collapse can be foundin the JRC Scientific and Technical Report [6]

A few years ago at the ELSA laboratory a reinforced concrete flat-slab framebuilding was tested to evaluate its safety against collapse (see [4]) First staticlinear and nonlinear analyses of the building under column removals were performedand then several columns of the building were demolished one after the otherto observe the building behaviour This experiment has shown not only that thestructure survived the demolition of two central columns but also how challengingthe structural testing against progressive collapse is Even though the columns weredemolished rather slowly using a concrete crunching machine still safety provisionsprevented the planned sequence of column removal from being followed

However buildings can be exposed to fast dynamic abnormal events suchas bomb explosions or impacts so the dynamic nature of the loading must beconsidered Therefore the purpose of this report is to re-evaluate the previouslymentioned frame building using linear and nonlinear dynamic analyses accordingto the alternate load path method In other words this study tries to answer thequestion what would have happened if the columns had been destroyed dynamically

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 4: JRC62663

7 Two central columns removed 78

8 Conclusions 79

References 82

List of Figures 83

List of Tables 88

A Photos from experimental destroying of columns 90

1 Introduction

Progressive collapse of structures occurs when a local failure triggers successivefailures and leads to the total collapse or a collapse disproportionate to the originalcause There were a few world-wide known examples of progressive collapses suchas the partial collapse of the Ronan Point residential apartment building (London1968) the major collapse of the Alfred P Murrah Federal Building (Oklahoma 1995)etc The first progressive collapse regulatory documents followed the Ronan Pointpartial collapse and were included into the British standards In turn after the totalcollapse of the World Trade Center towers many research activities lead to moredetailed guidelines on designing and preventing progressive collapses (eg [5] [2][8])

There are basically two approaches when dealing with the evaluation andprevention of progressive collapses in a given structure The first indirect approachconsists in ensuring that the structure satisfies prescriptive design rules (such asrequirements on structural integrity and ductility or the presence of vertical andhorizontal ties) The second direct approach uses two possibilities depending onwhether local failure is allowed or not If local failure is allowed then the structuremust be verified using the alternate load path method in which a load-bearingelement is removed from the structure If no local failure is allowed then keyelements must be designed to sustain a notional accidental action More detailedinformation on the state-of-the-art in the field of progressive collapse can be foundin the JRC Scientific and Technical Report [6]

A few years ago at the ELSA laboratory a reinforced concrete flat-slab framebuilding was tested to evaluate its safety against collapse (see [4]) First staticlinear and nonlinear analyses of the building under column removals were performedand then several columns of the building were demolished one after the otherto observe the building behaviour This experiment has shown not only that thestructure survived the demolition of two central columns but also how challengingthe structural testing against progressive collapse is Even though the columns weredemolished rather slowly using a concrete crunching machine still safety provisionsprevented the planned sequence of column removal from being followed

However buildings can be exposed to fast dynamic abnormal events suchas bomb explosions or impacts so the dynamic nature of the loading must beconsidered Therefore the purpose of this report is to re-evaluate the previouslymentioned frame building using linear and nonlinear dynamic analyses accordingto the alternate load path method In other words this study tries to answer thequestion what would have happened if the columns had been destroyed dynamically

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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1 Introduction

Progressive collapse of structures occurs when a local failure triggers successivefailures and leads to the total collapse or a collapse disproportionate to the originalcause There were a few world-wide known examples of progressive collapses suchas the partial collapse of the Ronan Point residential apartment building (London1968) the major collapse of the Alfred P Murrah Federal Building (Oklahoma 1995)etc The first progressive collapse regulatory documents followed the Ronan Pointpartial collapse and were included into the British standards In turn after the totalcollapse of the World Trade Center towers many research activities lead to moredetailed guidelines on designing and preventing progressive collapses (eg [5] [2][8])

There are basically two approaches when dealing with the evaluation andprevention of progressive collapses in a given structure The first indirect approachconsists in ensuring that the structure satisfies prescriptive design rules (such asrequirements on structural integrity and ductility or the presence of vertical andhorizontal ties) The second direct approach uses two possibilities depending onwhether local failure is allowed or not If local failure is allowed then the structuremust be verified using the alternate load path method in which a load-bearingelement is removed from the structure If no local failure is allowed then keyelements must be designed to sustain a notional accidental action More detailedinformation on the state-of-the-art in the field of progressive collapse can be foundin the JRC Scientific and Technical Report [6]

A few years ago at the ELSA laboratory a reinforced concrete flat-slab framebuilding was tested to evaluate its safety against collapse (see [4]) First staticlinear and nonlinear analyses of the building under column removals were performedand then several columns of the building were demolished one after the otherto observe the building behaviour This experiment has shown not only that thestructure survived the demolition of two central columns but also how challengingthe structural testing against progressive collapse is Even though the columns weredemolished rather slowly using a concrete crunching machine still safety provisionsprevented the planned sequence of column removal from being followed

However buildings can be exposed to fast dynamic abnormal events suchas bomb explosions or impacts so the dynamic nature of the loading must beconsidered Therefore the purpose of this report is to re-evaluate the previouslymentioned frame building using linear and nonlinear dynamic analyses accordingto the alternate load path method In other words this study tries to answer thequestion what would have happened if the columns had been destroyed dynamically

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 6: JRC62663

(eg as it could be in the case of a bomb explosion or other accidental action) Forcompleteness there is also included in this report a comparison between the dynamicanalysis and the previously performed static analysis

2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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2 Description of the structure

The structure was a 3-storey 2-bay reinforced concrete frame building with a024m thick slab (Figure 21) The structure contained two main frames connectedtogether with transverse beams (Figure 22) The girder beams were 1m wide and024m high The slab had the same height (020m thick and 004m topping) as thebeams The frames were supported by square columns with the size of 04times 04mIn each frame there existed an eccentricity of 02m between the axes of beams andcolumns Because of the reduced beam height they had quite high reinforcementon both sides with only some rebars anchored to the column joints

The structure was designed for medium seismicity (which corresponds to a 025gpeak ground acceleration) however some detailing rules were intentionally violatedThis applied to the mentioned eccentricity between beams and columns as well as tothe lack of design for ductility The details of reinforcement are shown in Figures 23and 24

Despite these drawbacks the structure had survived the design earthquaketesting at the reaction wall facility with minor damage and had been transportedout of the laboratory for demolition Taking this opportunity it was decided tostudy its safety margins against progressive collapse

21 Materials

The materials of the structure were C2530 concrete and S500 steel In additionlaboratory tests were performed on cubes of concrete and on three specimens of eachrebar diameter The results are presented in Tables 21 and 22

Table 21 Concrete strength (mean values)

Origin of the sample fcm [MPa] fck [MPa]

1st floor columns 3436 31451st floor slab 3598 33082nd floor columns 3687 33972nd floor slab 3380 30903rd floor columns 3342 30513rd floor slab 3961 3671

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 8: JRC62663

6 Chapter 2 Description of the structure

Figure 21 Front view

Figure 22 Floor plan

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 9: JRC62663

Materials 7

Figure 23 Elevation and column rebars

Figure 24 Beam rebars

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 10: JRC62663

8 Chapter 2 Description of the structure

Table 22 Steel strength (mean values)

rebar size fy [MPa] ft [MPa] εu []

8mm 53480 61036 91210mm 56553 65976 100114mm 53286 64053 106016mm 53116 64190 111418mm 53513 64340 101020mm 52456 64256 1107

22 Resistance of the frame elements

This section presents the calculated values of resistance for both beams andcolumns against which the computed internal forces will be checked

Assuming that in beams failure is due to bending (neglecting axial and shearforces) the approximated beam moment resistance is calculated as

Mr = 085Asfsd (21)

where As is the area of reinforcement bars in the beam cross-section fs is thecharacteristic value of strength of steel (5246MPa) and d is the distance fromthe centre of reinforcement to the extreme compressed concrete fibers of the beamcross-section The assumption of neglecting the axial forces is justified becauseusually the axial forces in beams are relatively small and increasedecrease thebending moment resistance only marginally (see an example of the interactiondiagram for a type 1 beam in Figure 25)

For columns the pure axial resistance is calculated as

Nr = Acfc + Asfs (22)

where Ac is the area of the concrete cross-section and fc is the strength of concrete incompression (328MPa) Their approximated pure bending resistance is calculatedvia Eq (21)

The calculated resistance for the beams are presented in Table 23 and for thecolumns in Table 24 (a - longer bay b - shorter bay)

Note that for those beams which will undergo bending reversal after the columnremoval two values of resistance are listed in Table 23 (positive and negativemoment)

However for columns the influence of axial force on bending moment resistancecannot be neglected therefore the actual bending moment resistance is obtainedfrom the interaction diagrams plotted in Figs 26-29 These interaction diagramswere calculated with SAP 2000 for four types of column cross-sections The usageof these interaction diagrams is illustrated in Chapter 41 (Fig 44)

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 11: JRC62663

Resistance of the frame elements 9

minus50 0 50 100 150 200 250 300 350 400minus4000

minus2000

0

2000

4000

6000

8000

10000PminusM3 interaction diagram for the beam type 1

bending moment [kNm]

axia

l for

ce [k

N]

Figure 25 Interaction diagram for a type 1 beam

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 14

bending moment [kNm]

axia

l for

ce [k

N]

Figure 26 Interaction diagram for a column with rebars φ14

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 12: JRC62663

10 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 16

bending moment [kNm]

axia

l for

ce [k

N]

Figure 27 Interaction diagram for a column with rebars φ16

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000PminusM3 interaction diagram for a column with rebars φ 18

bending moment [kNm]

axia

l for

ce [k

N]

Figure 28 Interaction diagram for a column with rebars φ18

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 13: JRC62663

Resistance of the frame elements 11

Table 23 Resistance of beams

Beam Mr [kNm] Mr [kNm]

Floors 1-2

a - left 197087a - middle 92349a - right 225242 176815b - left 225242 176815b - middle 92349b - right 112621

Floor 3

a - left 168932a - middle 92349a - right 197087 148660b - left 197087 148660b - middle 92349b - right 112621

Table 24 Resistance of columns

Column Nr [kN] Mr [kNm]

Floor 1

1 5836481 968212 6170202 1512823 5836481 96821

Floor 2

1 5836481 968212 5836481 968213 5697431 74128

Floor 3

1 5994072 1225392 5836481 968213 5697431 74128

The internal forces will be obtained from a FE calculation using the commercialsoftware SAP 2000 and in the most loaded cross-sections they will be compared tothe corresponding resistance values

12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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12 Chapter 2 Description of the structure

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000PminusM3 interaction diagram for a column with rebars φ 20

bending moment [kNm]

axia

l for

ce [k

N]

Figure 29 Interaction diagram for a column with rebars φ20

23 Summary of the previous analyses and experiment for

progressive collapse

The structure described earlier was first tested pseudodynamically against adesign earthquake The results reported in [7] showed that the structure sufferedminor damage Then the structure was devoted to controlled demolition with thegoal of investigating its safety against collapse

However before the experiment the structure was analysed numerically usingthree different FE programs First the linear static analyses were performed inSAP 2000 using the geometrical and material properties of the virgin structure andapplying vertical loads corresponding to self-weight The self-weight of the structurewas represented by uniformly distributed loads to account for the one-way structuralscheme of the slabs In this analysis the most significant cases of column removalwere presented namely the removal of a central column in the first frame andthen the removal of both central columns The conclusions were drawn from thecomparison between the resulting internal forces and the computed yield momentsthe structure would have survived the annihilation of any single column with minoryielding while it would have collapsed after the removal of both central columnsbecause the distribution of bending moments resulted to be far beyond the yieldlimit

Then a nonlinear static analyses were carried out in ADINA The mechanicalproperties of the cross-sections were specified as monotonic moment-curvaturerelationships The removal of a single central column resulted in yielding in the firstframe at the ends of the beams of the first two stories and at the top of the external

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 15: JRC62663

columns of the top storey However the maximum plastic curvature remained belowthe assumed ultimate curvature capacity The removal of both central columns gaveyielding in the whole structure and the plastic curvature demands reached theirmaximum at the top of the columns of the top storey The total curvature demandwas in this case much higher than the capacity therefore the results indicated thatthe structure would have collapsed

The nonlinear static analyses were repeated in IDARC2D in order to include thesoftening branch in moment-curvature relationship and the results suggested thatthe structure would have survived even if both central columns were removed

The experimental part involved the successive cutting of the columns In thefirst phase one central column was cut out As can be seen in Figures A1 andA2 in Annex A the building withstood the lack of this load-bearing member Inthe second phase the other central column was removed and again the structuresurvived (see Figures A3 and A4) Then there was concern that the building wouldcollapse in an uncontrolled manner (after a complete removal of another column)therefore for safety reasons it was decided to progressively destroy two externalcolumns to provoke a pancake-type collapse (see Figures A5ndashA9)

In the context of the experimental investigation it is worth mentioning thatinitially another reinforced concrete frame building (see [9]) was planned to bedestroyed and tested against collapse Unfortunately the first stages of demolitionhad activated large vibrations in the whole building and for safety reasons again itwas decided to stop the procedure and destroy the building in a safer way Thus noexperimental information was obtained about the potential progressive collapse ofthat building This example also showed that it is very difficult to experimentallyassess the safety against collapse On the contrary numerical analyses are easierand allow to consider different scenarios

It should be noted however that the above-presented results both numerical andexperimental took into account only the static behaviour of the structure Thusa question arises would the structure have survived if a columncolumns had beendestroyed dynamically In the following chapters the results of numerical linear andnonlinear static and dynamic analyses are presented to give a preliminary answerto this question

3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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3 Finite element model in SAP 2000

A finite element model of the analysed structure has been created in SAP 2000the element numbers (Figure 31) and node numbers (Figure 32) will be often usedin the sequel to display the numerical results The first longer bay in x-direction isreferred to as rsquoarsquo-bay while the second one as rsquobrsquo-bay

In this report three scenarios are considered sudden removal of a central columna left corner column and finally a right corner column (see Figure 33)

Only the self-weight was considered at the moment of demolition This was equalto 35 kNm2 (actual concrete structure weight) plus 20 kNm2 representing severalpermanent fixtures on the structure The self-weight was modelled as a uniformlydistributed linear load applied to the girders (see Figure 34) to account for theone-way behaviour of the concrete slabs The column to be removed is replaced bythe corresponding reaction forces at the appropriate node (see Figure 35)

In dynamic analyses the simulation of the column removal is performed bysuddenly cancelling the reaction forces standing for the column in practice a similarset of forcesmoments is applied in the opposite direction (see Figure 36) The rateof the column removal is specified by a time function also presented in Figure 36(linear ramp to maximum value) For actual bomb explosions the time in which astructural member is destroyed is very short (some milliseconds) In the presentedFE calculations the removal time is chosen close to zero (5ms) which means a quasiinstantaneous removal The dynamic effects of the removal rate on the dynamicresponse of the structure were analysed in Report [6] and the results showed thatthe most unfavourable dynamic effects occur when the column is destroyed within atime close to zero (below 5ms) The dynamic computations are performed startingfrom the equilibrium position of the intact structure under gravity loads (zero initialvelocities) and assuming a 5 viscous damping

Figure 37 presents the summary of the loading case used in SAP 2000calculations

15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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15

Figure 31 Finite element model of the analysed frame in SAP 2000 - element numbers

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 18: JRC62663

16 Chapter 3 Finite element model in SAP 2000

Figure 32 Frame model in SAP 2000 - node numbers

Figure 33 Analysed scenarios of column removal

17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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17

Figure 34 Loads on the frame self weight

Figure 35 Loads on the frame reaction from the actual column at node 48

18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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18 Chapter 3 Finite element model in SAP 2000

Figure 36 Loads on the frame - simulation of the column removal (from SAP 2000)

Figure 37 Loads on the frame - load case (from SAP 2000)

4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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4 Linear static analysis

This chapter addresses the linear static analyses of the intact structure and ofthe three scenarios of column removals mentioned before (see Figure 33) Theseanalyses have already been performed and the results reported in [4] However tomake this report self-contained they have been reproduced to compare with thedynamic analyses

The results obtained from these static computations are compared with thestructural resistances using the so called demand-resistance ratios (DRR) A localDRR is defined in each section as

DRR =

MmaxMr in beams (bending moment only)

NmaxNr in bars (axial force only)

MmaxMr(N) in columns (combined bending moment and axial force)

(41)where Mmax and Nmax are the maximum moment and axial force acting on thesection while Mr and Nr are the bending moment and axial resistances of the sectionrespectively The global DRR is taken as the maximum local DRR over the structureie DRRmax For reinforced concrete structures both [5] and [2] specify that thevalue of 200 for the demand-resistance ratio should not be exceeded otherwise thestructure is deemed as prone to progressive collapse

41 Before demolition

The results in this phase concern the frames in the intact state ie all elementsare present as compared to the subsequent phases where one or more columns aredestroyed

The results being exactly the same for both frames are displayed only onceFigures 41ndash43 display the internal forces (bending moments shear forces andaxial forces) in both frames while their values in the most representativecriticalcross-sections are given in Table 41 for beams and in Table 42 for columns Theloading corresponds to the above-mentioned self-weight of (35 + 2) kNm2 In theTables the resultant internal forces are given at the different cross-sections (l - leftmid - midspan r - right) of each bay (a - longer bay b - shorter bay) togetherwith the ratios between the resultant internal forces and the element resistances(demandresistance ratio - DRR) Note that the Mr values in these Tables are

20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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20 Chapter 4 Linear static analysis

obtained from the interaction diagrams (Figs 26-29) accordingly As an exampleFig 44 shows how the value of Mr is obtained for the first-floor central columns(with rebars φ20) under the axial force Ns = 26720 kN The maximum values ofdemandresistance ratios are highlighted the most loaded cross-sections are themidspan of the left beams on the third floor (DRR = 3253) and the top of theleft column on the third floor (DRR = 2933) but their demand-resistance ratiosare relatively small

Figure 41 Bending moments original structure

Table 41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2

Frame 1 and 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3004 5252 2930 1108 1784floor 2 4959 2718 5198 2307 1146 2331floor 1 4722 2826 5218 2690 1128 1984

MsMr []

floor 3 2566 3253 2665 1487 1200 1584floor 2 2516 2943 2308 1024 1241 2070floor 1 2396 3060 2317 1194 1221 1762

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 23: JRC62663

Before demolition 21

Figure 42 Shear forces original structure

Figure 43 Axial forces original structure

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 24: JRC62663

22 Chapter 4 Linear static analysis

0 50 100 150 200 250 300 350 400minus1000

0

1000

2000

3000

4000

5000

6000

7000

Finding Mr(N

s) from the interaction diagram (rebars φ 20)

bending moment [kNm]

axia

l for

ce [k

N]

Ns = 26720kN

Mr=21203kNm

Figure 44 Interaction diagram for a column with rebars φ20

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 25: JRC62663

Before demolition 23

Table 42 Axial forces and bending moments in columns no column removal comparisonwith resistance frames 1 and 2

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 5045 9102 3178floor 3 bot 5045 9102 3178floor 2 top 10202 17799 6649floor 2 bot 10202 17799 6649floor 1 top 15317 26720 9938floor 1 bot 15317 26720 9938

Ms [kNm]

floor 3 top 4334 2321 1784floor 3 bot 2806 1689 1345floor 2 top 2153 1202 986floor 2 bot 2691 1482 1175floor 1 top 2031 1046 809floor 1 bot 909 598 487

Mr(Ns) [kNm]

floor 3 top 14778 12638 9124floor 3 bot 14778 12638 9124floor 2 top 12830 14057 9738floor 2 bot 12830 14057 12209floor 1 top 13671 21203 12784floor 1 bot 13671 21203 12784

MsMr(Ns) []

floor 3 top 2933 1837 1955floor 3 bot 1899 1336 1474floor 2 top 1678 855 1013floor 2 bot 2097 1054 962floor 1 top 1486 493 633floor 1 bot 665 282 381

24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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24 Chapter 4 Linear static analysis

42 One central column removed

In phase 1 a central column in the first frame is removed Figures 45-48display the bending moment and axial force distributions for both frames and thecorresponding values are given in Tables 43-45

Figure 45 Bending moments linear static analysis central column removed frame 1

The linear static analysis shows that the most loaded cross-sections are in thefirst frame namely the right-end of the right beam on the second floor (DRR =12372) and the top of the right column on the third floor (DRR = 10777) Thevertical displacement at node 48 is equal to 00167m

As stated in [4] these results indicate only minor yielding so the structureis not susceptible to collapse statically However according to guidelines in[2] and [5] a structure is susceptible to progressive collapse (dynamically) whenits demand-resistance ratio exceeds 200 provided that the permanent loads aremultiplied by a factor of 2 (to accounts for dynamic effects) in the computationof internal forces Therefore in this case if the loads were doubled thedemand-resistance ratios would exceed 200 and the structure would be deemedas susceptible to progressive collapse dynamically

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 27: JRC62663

One central column removed 25

Figure 46 Axial forces linear static analysis central column removed frame 1

Figure 47 Bending moments linear static analysis central column removed frame 2

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 28: JRC62663

26 Chapter 4 Linear static analysis

Figure 48 Axial forces linear static analysis central column removed frame 2

Table 43 Bending moments in beams central column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 13001 3593 4594 7746 1826 11025floor 2 15060 2851 5169 9912 1454 13933floor 1 13813 3410 5041 9130 1220 13621

MsMr []

floor 3 7696 3891 3090 5211 1977 9789

floor 2 7641 3087 2923 5606 1574 12372floor 1 7009 3693 2851 5164 1321 12095

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4652 2992 4956 3369 1138 1285floor 2 5443 2704 4742 2947 1159 1665floor 1 5171 2814 4794 3307 1146 1332

MsMr []

floor 3 2754 3240 2515 1709 1232 1141floor 2 2762 2928 2105 1308 1255 1478floor 1 2624 3047 2128 1468 1241 1183

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 29: JRC62663

One central column removed 27

Table 44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 8130 1037 8158floor 3 bot 8130 1037 8158floor 2 top 16699 168 17584floor 2 bot 16699 168 17584floor 1 top 25039 - 26737floor 1 bot 25039 - 26737

Ms [kNm]

floor 3 top 12979 3024 10781floor 3 bot 8539 1854 7740floor 2 top 6589 2784 6040floor 2 bot 9646 3955 6030floor 1 top 4152 - 7443floor 1 bot 3179 - 2519

Mr(Ns) [kNm]

floor 3 top 15261 11229 10004floor 3 bot 15261 11229 10004floor 2 top 13886 11078 11669floor 2 bot 13886 11078 14024floor 1 top 15184 - 15449floor 1 bot 15184 - 15449

MsMr(Ns) []

floor 3 top 8505 2693 10777floor 3 bot 5595 1651 7737floor 2 top 4745 2513 5176floor 2 bot 6947 3570 4300floor 1 top 2734 - 4818floor 1 bot 2094 - 1631

28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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28 Chapter 4 Linear static analysis

Table 45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5147 9234 2944floor 3 bot 5147 9234 2944floor 2 top 10461 18300 6088floor 2 bot 10461 18300 6088floor 1 top 15722 27394 9060floor 1 bot 15722 27394 9060

Ms [kNm]

floor 3 top 4674 1715 1529floor 3 bot 2931 1279 1201floor 2 top 2444 620 618floor 2 bot 3031 866 797floor 1 top 2154 756 682floor 1 bot 1578 342 341

Mr(Ns) [kNm]

floor 3 top 14794 12661 9083floor 3 bot 14794 12661 9083floor 2 top 12875 14135 9638floor 2 bot 12875 14135 12111floor 1 top 13734 15551 12630floor 1 bot 13734 15551 12630

MsMr(Ns) []

floor 3 top 3159 1355 1683floor 3 bot 1981 1010 1322floor 2 top 1898 439 641floor 2 bot 2354 613 658floor 1 top 1568 486 540floor 1 bot 1149 220 270

One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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One left corner column removed 29

43 One left corner column removed

Many current progressive collapse provisions in codes standards and guidelines(eg [3] [5] [2]) require that the load-bearing elements are removed anywhere in thestructure one at a time and check if progressive collapse could occur Therefore inthis section a left corner column is removed from the first frame

Figures 49-412 provide the internal force distribution in both frames whereasTables 46-48 show their values in the representative cross-sections The maximumdemand-resistance ratio are reached on the third floor at the right-ends of the leftbeam (DRR = 13264) and at the top of the right columns (DRR = 9231) Thevertical displacement at node 25 equals 00552m Therefore according to the rulesof thumbs mentioned earlier a progressive collapse is unlikely under static conditions(DRR lt 200) but is possible under dynamic conditions (DRR gt 200)

Figure 49 Bending moments linear static analysis left corner column removed frame 1

30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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30 Chapter 4 Linear static analysis

Figure 410 Axial forces linear static analysis left corner column removed frame 1

Figure 411 Bending moments linear static analysis left corner column removedframe 2

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 33: JRC62663

One left corner column removed 31

Figure 412 Axial forces linear static analysis left corner column removed frame 2

Table 46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 8735 2304 19719 14955 028 8082floor 2 11237 2657 21515 15057 774 9675floor 1 10310 1932 22038 11961 407 5844

MsMr []

floor 3 5171 2495 13264 10060 030 7176floor 2 5702 2877 12168 8516 838 8591floor 1 5231 2092 12464 6765 441 5189

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5456 2931 4274 4211 1159 402floor 2 6419 2683 3807 4302 1186 256floor 1 6165 2766 3896 4544 1176 033

MsMr []

floor 3 3230 3174 2169 2137 1255 357

floor 2 3257 2905 1690 1910 1284 227floor 1 3128 2995 1730 2017 1273 029

32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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32 Chapter 4 Linear static analysis

Table 47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 455 19164 2294floor 3 bot 455 19164 2294floor 2 top 194 39458 5012floor 2 bot 194 39458 5012floor 1 top - 57963 5998floor 1 bot - 57963 5998

Ms [kNm]

floor 3 top 8192 4378 8278floor 3 bot 4221 3076 5824floor 2 top 6667 3040 4125floor 2 bot 9839 730 8284floor 1 top - 9086 2532floor 1 bot - 2836 3296

Mr(Ns) [kNm]

floor 3 top 13984 14270 8968floor 3 bot 13984 14270 8968floor 2 top 11082 17429 9448floor 2 bot 11082 17429 11924floor 1 top - 26108 12096floor 1 bot - 26108 12096

MsMr(Ns) []

floor 3 top 5858 3068 9231floor 3 bot 3018 2156 6494floor 2 top 6016 1744 4366floor 2 bot 8878 419 6947floor 1 top - 3480 2093floor 1 bot - 1086 2725

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 35: JRC62663

One left corner column removed 33

Table 48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 5394 9418 2513floor 3 bot 5394 9418 2513floor 2 top 11027 18667 4966floor 2 bot 11027 18667 4966floor 1 top 16603 28079 7303floor 1 bot 16603 28079 7303

Ms [kNm]

floor 3 top 4913 449 598floor 3 bot 3055 436 720floor 2 top 3014 589 190floor 2 bot 3507 460 107floor 1 top 2187 459 590floor 1 bot 2871 1763 1662

Mr(Ns) [kNm]

floor 3 top 14833 12693 9007floor 3 bot 14833 12693 9007floor 2 top 12974 14192 9440floor 2 bot 12974 14192 11915floor 1 top 13871 21416 12324floor 1 bot 13871 21416 12324

MsMr(Ns) []

floor 3 top 3312 354 664floor 3 bot 2060 343 799floor 2 top 2323 415 201floor 2 bot 2703 324 090floor 1 top 1577 214 479floor 1 bot 2070 823 1349

34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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34 Chapter 4 Linear static analysis

44 One right corner column removed

The last case deals with the removal of a right corner column from the firstframe Figures 413-416 present the bending moment and axial force diagramswhile Tables 49-411 give their values and demand-resistance ratios This case issimilar to the previous one and is more favourable because the span of the rightbay is shorter The demand-resistance ratios are far below 100 in all membersThe maximum DRR values are 6623 for beams and 392 for columns and thevertical displacement at node 69 is equal to 00142m In this case the linear staticcalculation indicates that the structure would not collapse neither statically (DRR lt100) nor dynamically (DRR lt 200)

Figure 413 Bending moments linear static analysis right corner column removedframe 1

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 37: JRC62663

One right corner column removed 35

Figure 414 Axial forces linear static analysis right corner column removed frame 1

Figure 415 Bending moments linear static analysis right corner column removedframe 2

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 38: JRC62663

36 Chapter 4 Linear static analysis

Figure 416 Axial forces linear static analysis right corner column removed frame 2

Table 49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2

Frame 1 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 371 2688 9846 8939 725 3458floor 2 364 2610 10008 9416 1130 4746floor 1 1672 2596 8729 10436 540 4585

MsMr []

floor 3 220 2911 6623 6013 785 3070floor 2 185 2826 5660 5325 1224 4214floor 1 848 2811 4937 5902 585 4071

Frame 2 Ms [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 3771 3033 5756 2220 1067 2576floor 2 4160 2739 5955 1234 1127 3442floor 1 3961 2853 5925 1667 1094 3076

MsMr []

floor 3 2232 3284 2921 1126 1155 2287floor 2 2111 2966 2644 548 1220 3056floor 1 2010 3089 2631 740 1185 2731

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 39: JRC62663

One right corner column removed 37

Table 410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1

Frame 1 Ns [kN]

Column 1 2 3

floor 3 top 3619 13341 366floor 3 bot 3619 13341 366floor 2 top 7209 27137 290floor 2 bot 7209 27137 290floor 1 top 11230 40731 -floor 1 bot 11230 40731 -

Ms [kNm]

floor 3 top 252 1113 3194floor 3 bot 046 755 1511floor 2 top 539 407 3050floor 2 bot 1591 1695 4351floor 1 top 3186 3260 -floor 1 bot 1698 1684 -

Mr(Ns) [kNm]

floor 3 top 14533 13363 8628floor 3 bot 14533 13363 8628floor 2 top 12307 15511 8614floor 2 bot 12307 15511 11099floor 1 top 13009 23402 -floor 1 bot 13009 23402 -

MsMr(Ns) []

floor 3 top 173 833 3702floor 3 bot 032 565 1751floor 2 top 438 262 3541

floor 2 bot 1293 1093 3920floor 1 top 2449 1393 -floor 1 bot 1305 720 -

38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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38 Chapter 4 Linear static analysis

Table 411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2

Frame 2 Ns [kN]

Column 1 2 3

floor 3 top 4867 8904 3554floor 3 bot 4867 8904 3554floor 2 top 9765 17328 7571floor 2 bot 9765 17328 7571floor 1 top 14635 25965 11388floor 1 bot 14635 25965 11388

Ms [kNm]

floor 3 top 3891 3331 2313floor 3 bot 2642 2359 1642floor 2 top 1672 2176 1615floor 2 bot 2178 2535 1804floor 1 top 1861 1581 1037floor 1 bot 725 1888 1627

Mr(Ns) [kNm]

floor 3 top 14750 12603 9191floor 3 bot 14750 12603 9191floor 2 top 12754 13984 9900floor 2 bot 12754 13984 12370floor 1 top 13564 15329 13037floor 1 bot 13564 15329 13037

MsMr(Ns) []

floor 3 top 2638 2643 2517floor 3 bot 1791 1872 1787floor 2 top 1311 1556 1631floor 2 bot 1708 1813 1458floor 1 top 1372 1031 795floor 1 bot 535 1232 1248

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 41: JRC62663

5 Linear dynamic analysis

This chapter presents the results of the three scenarios of column removal usinglinear dynamic analysis The advantage of this kind of calculations is that dynamiceffects are inherently incorporated in the analysis as opposed to an a priori assumeddynamic factor to be applied on the results of the static analysis Since it providesa more realistic distribution of the internal forces over the structure the lineardynamic analysis is expected to give a more reliable estimate of the actual maximumdemand-resistance ratio (DRRmax) characterising the structural robustness againstprogressive collapse Furthermore the actual dynamic factor that should be appliedto the static analysis results can be computed a posteriori It is however worthmentioning that the notion of dynamic factor is well-defined only for a single degreeof freedom system where all quantities (force displacement DRR etc) lead to thesame dynamicstatic ratio In a multi degree of freedom system different definitionscan be adopted which lead to different values of the dynamic factor namely

bull the ratio of the dynamic and static maximum deflection at the top of the removedcolumn

bull the maximum ratio of the dynamic and static local DRRbull the ratio of the dynamic and static DRRmax

Despite the apparent soundness of the first two definitions only the third definitionseems to be correct in the following sense if this dynamic factor is applied to thestatic results the output of the dynamic analysis is recovered in terms of robustness(value of DRRmax) This will be confirmed by the results of the linear and nonlineardynamic analyses

The procedure used in the calculations has been presented in Chapter 3 Insummary the following steps were carried out in SAP 2000

bull build a FE modelbull find the reaction forces of a column to be removed under the self-weight loading

(see Figure 34)bull remove this column from the FE model and apply these reactions in its place

(Figure 35)bull apply these reaction forces again but in the opposite direction using a linear

ramp function (Figure 36)bull perform linear time history analysis with initial conditions and 5 critical

damping (Figure 37)

The results obtained from these dynamic computations (time histories of internal

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 42: JRC62663

40 Chapter 5 Linear dynamic analysis

forces) are compared with the resistances (using Eq (41) defined in Chapter 4) andwith the corresponding static responses obtained in the previous chapter

51 One central column removed

This section reports on the response of the structure to the sudden removalof the central column in the first frame Figures 51-54 show the envelopes ofthe internal forces (bending moments and axial forces) in both frames while thecorresponding maximum values for beams and columns are presented in Tables51-53 respectively First of all as could be expected Frame 2 is significantly lessaffected than Frame 1 where the column was removed a fact that can be explained bythe one-way behaviour of the flat-slab frame The most critical sections in terms ofdemand-resistance ratio are the right-end of the right beam on the first floor (DRR =21254) and the top of the right column on the third floor (DRR = 15983) Sincethe demand-resistance ratio for beams exceeded the 200 threshold the building issusceptible to progressive collapse

As for local dynamic factors in beams the maximum values are reached at theright-end of the left beam at the third floor in the first frame but also at the right-endof the right beam at the first floor in the second frame (222) while in columns themaximum dynamic factors are much larger and reach 358 and 871 in the first andsecond frame respectively This fact demonstrates that it is difficult to draw anyconclusion from the local dynamic factors because they are highly heterogeneousthroughout the structure especially in columns where the static and dynamic forcesare quite different

In Figures 55-56 are plotted the time history of the displacement at Node 48 andthe bending moments at the most loaded section From the maximum displacementof the dynamic (00268m) and static (00167m) responses at node No 48 a ratioof 160 is found which can be interpreted as a global dynamic factor

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 43: JRC62663

One central column removed 41

Figure 51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1

Figure 52 Envelope of axial forces linear dynamic analysis central column removedframe 1

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 44: JRC62663

42 Chapter 5 Linear dynamic analysis

Figure 53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2

Figure 54 Envelope of axial forces linear dynamic analysis central column removedframe 2

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 45: JRC62663

One central column removed 43

0 02 04 06 08 1 12minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

static value

Figure 55 Vertical deflection at node No 48 central column removed

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 56 Bending moment at the most critical section central column removed

44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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44 Chapter 5 Linear dynamic analysis

Table 51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 19021 5212 10204 13670 2593 17417floor 2 22048 4021 9850 18072 2116 22894floor 1 18613 4957 9626 17665 1592 23937

MdMr []

floor 3 11260 5644 6864 9195 2808 15465floor 2 11187 4354 5571 10221 2291 20328

floor 1 9444 5368 5444 9991 1724 21254

MdMs ndash local dynamic factor

floor 3 146 145 222 176 142 158floor 2 146 141 191 182 146 164floor 1 135 145 191 193 130 176

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 5625 3038 5408 4662 1201 1879floor 2 6853 2741 5526 4940 1199 2771floor 1 6392 2870 5863 4962 1201 2951

MdMr []

floor 3 3330 3290 2744 2365 1301 1668

floor 2 3477 2968 2453 2193 1298 2460floor 1 3243 3108 2603 2203 1301 2620

MdMs ndash local dynamic factor

floor 3 121 102 109 138 106 146floor 2 126 101 117 168 103 166floor 1 124 102 122 150 105 222

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 47: JRC62663

One central column removed 45

Table 52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 10531 9102 11844floor 3 bot 10559 9102 11876floor 2 top 22088 17799 26265floor 2 bot 22110 17799 26291floor 1 top 32819 - 40671floor 1 bot 32827 - 40682

Mmax

d [kNm]

floor 3 top 18997 6752 17027floor 3 bot 13429 4198 11830floor 2 top 8740 7150 10933floor 2 bot 15738 9172 9009floor 1 top 6694 - 15073floor 1 bot 9268 - 10247

Nd [kN] for Mmax

d

floor 3 top 10520 1224 11833floor 3 bot 10544 1654 11771floor 2 top 22088 1137 26200floor 2 bot 22074 2218 26121floor 1 top 22994 - 39956floor 1 bot 22117 - 40376

Mr(Nd) [kNm]

floor 3 top 15634 11262 10653floor 3 bot 15638 11337 10642floor 2 top 14725 11247 13093floor 2 bot 14723 11436 15353floor 1 top 14866 - 17507floor 1 bot 14729 - 17572

Mmax

d Mr(Nd) []

floor 3 top 12151 5995 15983floor 3 bot 8587 3703 11116floor 2 top 5935 6357 8350floor 2 bot 10689 8020 5868floor 1 top 4503 - 8610floor 1 bot 6292 - 5831

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 143 223 148floor 3 bot 153 224 144floor 2 top 125 253 161floor 2 bot 154 225 136floor 1 top 165 - 179floor 1 bot 300 - 358

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 48: JRC62663

46 Chapter 5 Linear dynamic analysis

Table 53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5391 11316 3232floor 3 bot 5393 11322 3245floor 2 top 11120 22165 6768floor 2 bot 11122 22176 6777floor 1 top 16744 32748 10093floor 1 bot 16744 32752 10097

Mmax

d [kNm]

floor 3 top 5648 2460 2034floor 3 bot 3605 2064 1868floor 2 top 3436 1366 1266floor 2 bot 4201 1641 1309floor 1 top 3338 1920 1607floor 1 bot 4195 3124 2928

Nd [kN] for Mmax

d

floor 3 top 5388 7902 3171floor 3 bot 5288 9026 3047floor 2 top 10675 17892 5970floor 2 bot 11112 17298 6624floor 1 top 16115 32484 9465floor 1 bot 16182 25157 7987

Mr(Nd) [kNm]

floor 3 top 14832 12428 9123floor 3 bot 14816 12625 9101floor 2 top 12913 14072 9618floor 2 bot 12989 13979 12205floor 1 top 13795 22108 12701floor 1 bot 13805 20957 12443

Mmax

d Mr(Nd) []

floor 3 top 3808 1979 2230floor 3 bot 2433 1635 2053floor 2 top 2661 971 1316floor 2 bot 3234 1174 1073floor 1 top 2420 868 1265floor 1 bot 3039 1491 2353

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 121 146 133floor 3 bot 123 162 155floor 2 top 140 221 205floor 2 bot 137 192 163floor 1 top 154 179 234floor 1 bot 264 678 871

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 49: JRC62663

One left corner column removed 47

52 One left corner column removed

For the left corner column removal scenario the bending moments and axialforces are reported in Figures 57-510 and the corresponding maximum values aregiven in Tables 54-56 The maximum demand-resistance ratios are 19954 forbeams (right-end of the left beam of the third floor in Frame 1) and 17279 forcolumns (top of the right column of the third floor in Frame 1) In this scenariowe can notice even larger local dynamic factors (up to 2943) For this scenariothe maximum demand-resistance ratio (almost 200) is on the verge of treating thebuilding as acceptableunacceptable against progressive collapse

Figure 511 shows how the vertical displacement at node 25 varies in time Themaximum value is 0091m Similarly Figure 512 presents the bending moment timehistory at the most critical section The ratio between maximum linear dynamicdeflection and the deflection for the linear static analysis is 0091m00552m = 165

Figure 57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 50: JRC62663

48 Chapter 5 Linear dynamic analysis

Figure 58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1

Figure 59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 51: JRC62663

One left corner column removed 49

Figure 510 Envelope of axial forces linear dynamic analysis left corner columnremoved frame 2

0 02 04 06 08 1 12minus01

minus009

minus008

minus007

minus006

minus005

minus004

minus003

minus002

minus001

0Deflection at node 25

t [s]

defle

ctio

n [m

]

static value

Figure 511 Vertical deflection at node No 25 left corner column removed

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 52: JRC62663

50 Chapter 5 Linear dynamic analysis

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 512 Bending moment at most critical section left corner column removed

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 53: JRC62663

One left corner column removed 51

Table 54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 16328 3006 29663 24511 1203 16524floor 2 21070 3259 32591 26970 1197 21415floor 1 19123 2827 33274 24159 1190 18265

MdMr []

floor 3 9665 3255 19954 16488 1303 14672floor 2 10691 3529 18432 15253 1296 19015floor 1 9703 3061 18819 13663 1289 16218

MdMs ndash local dynamic factor

floor 3 187 130 150 164 4296 204floor 2 188 123 151 179 155 221floor 1 185 146 151 202 292 313

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 7480 3019 5415 6648 1275 2261floor 2 9657 2742 5824 8695 1273 4308floor 1 10142 2861 6114 9818 1338 5555

MdMr []

floor 3 4428 3269 2748 3373 1381 2008floor 2 4900 2969 2586 3860 1378 3825

floor 1 5146 3098 2714 4359 1449 4932

MdMs ndash local dynamic factor

floor 3 137 103 127 158 110 562floor 2 150 102 153 202 107 1683floor 1 165 103 157 216 114 16833

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 54: JRC62663

52 Chapter 5 Linear dynamic analysis

Table 55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5045 26622 6794floor 3 bot 5045 26631 6799floor 2 top 10202 55688 15369floor 2 bot 10202 55693 15373floor 1 top - 81440 21837floor 1 bot - 81442 21838

Mmax

d [kNm]

floor 3 top 15476 8019 16868floor 3 bot 8120 5534 11369floor 2 top 12542 7722 11453floor 2 bot 18387 6549 16477floor 1 top - 16403 6935floor 1 bot - 18753 21128

Nd [kN] for Mmax

d

floor 3 top 304 23399 6787floor 3 bot 977 23752 6775floor 2 top 124 38690 13644floor 2 bot 711 45469 15317floor 1 top - 65018 2894floor 1 bot - 65947 18956

Mr(Nd) [kNm]

floor 3 top 13957 14929 9762floor 3 bot 14074 14984 9760floor 2 top 11070 17310 10973floor 2 bot 11172 18365 13671floor 1 top - 27020 11554floor 1 bot - 27139 14237

Mmax

d Mr(Nd) []

floor 3 top 11088 5371 17279floor 3 bot 5770 3693 11649floor 2 top 11330 4461 10437floor 2 bot 16458 3566 12053floor 1 top - 6071 6002floor 1 bot - 6910 14840

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 189 175 187floor 3 bot 191 171 179floor 2 top 188 256 239floor 2 bot 185 851 173floor 1 top - 174 287floor 1 bot - 636 545

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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Page 55: JRC62663

One left corner column removed 53

Table 56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 7041 10048 3325floor 3 bot 7051 10049 3327floor 2 top 14218 20432 7249floor 2 bot 14226 20432 7251floor 1 top 21249 31206 11219floor 1 bot 21252 31207 11220

Mmax

d [kNm]

floor 3 top 6656 3597 2277floor 3 bot 3693 2159 1430floor 2 top 5979 5826 3879floor 2 bot 5540 5188 2995floor 1 top 4246 4092 2462floor 1 bot 9933 9865 9069

Nd [kN] for Mmax

d

floor 3 top 5792 10046 3325floor 3 bot 5547 10033 3327floor 2 top 13334 20312 1721floor 2 bot 12226 20401 1475floor 1 top 20605 31107 1465floor 1 bot 20575 31128 1399

Mr(Nd) [kNm]

floor 3 top 14895 12803 9150floor 3 bot 14857 12800 9151floor 2 top 13362 14448 8867floor 2 bot 13183 14462 11306floor 1 top 14494 21891 11304floor 1 bot 14489 21895 11293

Mmax

d Mr(Nd) []

floor 3 top 4469 2809 2489floor 3 bot 2486 1687 1563floor 2 top 4475 4032 4375floor 2 bot 4202 3587 2649floor 1 top 2929 1869 2178

floor 1 bot 6856 4506 8031

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 135 794 375floor 3 bot 121 492 196floor 2 top 193 972 2177floor 2 bot 155 1107 2943floor 1 top 186 873 455floor 1 bot 331 548 595

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 56: JRC62663

54 Chapter 5 Linear dynamic analysis

53 One right corner column removed

For the right corner column removal scenario the bending moments and axialforces are reported in Figure 513-516 and the corresponding maximum values aregiven in Tables 57-59 The maximum demand-resistance ratios are 8454 forbeams (right-end of the left beam on the third floor of Frame 1) and 6941 forcolumns (top of the right column on the second floor of Frame 1) In this case themaximum local dynamic factor is the largest observed so far (5934) which can beexplained by the small static bending moment 032 kNm (see Table 410)

Figure 517 shows the function of the vertical displacement at node 69 in timeThe maximum value is 00207m Figure 518 presents how the bending moment inthe most critical section varies in time The ratio between maximum linear dynamicdeflection and the deflection for the static analysis is 00207m00142m = 146

The results demonstrate that this is the most favourable failure scenario and thatthe structure bridges over the lacking column very efficiently In fact the structureremains in the elastic range

Figure 513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 57: JRC62663

One right corner column removed 55

Figure 514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1

Figure 515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 58: JRC62663

56 Chapter 5 Linear dynamic analysis

Figure 516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2

0 02 04 06 08 1 12minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 69

t [s]

defle

ctio

n [m

]

static value

Figure 517 Vertical deflection at node No 69 right corner column removed

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 59: JRC62663

One right corner column removed 57

0 02 04 06 08 1 12minus160

minus140

minus120

minus100

minus80

minus60

minus40

minus20Bending moment at leftminusend of element 83

t [s]

bend

ing

mom

ent [

kNm

]

static value

Figure 518 Bending moment at most critical section right corner column removed

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 60: JRC62663

58 Chapter 5 Linear dynamic analysis

Table 57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2

Frame 1 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4334 3039 12567 11057 1108 5104floor 2 4963 2746 12958 12471 1434 7379floor 1 4759 2859 11738 14551 1128 7746

MdMr []

floor 3 2566 3291 8454 7438 1200 4532floor 2 2518 2974 7329 7053 1553 6552floor 1 2415 3096 6639 8230 1221 6878

MdMs ndash local dynamic factor

floor 3 1168 113 128 124 153 148floor 2 1363 105 129 132 127 155floor 1 285 110 134 139 209 169

Frame 2 Md [kNm]

Beam a-left a-mid a-right b-left b-mid b-right

floor 3 4538 3148 7056 3186 1120 4515floor 2 5151 2804 7777 2592 1158 6168floor 1 4837 2932 7462 2831 1133 5396

MdMr []

floor 3 2686 3409 3580 1617 1213 4009

floor 2 2614 3036 3453 1151 1254 5477floor 1 2454 3175 3313 1257 1227 4791

MdMs ndash local dynamic factor

floor 3 120 104 123 144 105 175floor 2 124 102 131 210 103 179floor 1 122 103 126 170 104 175

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 61: JRC62663

One right corner column removed 59

Table 58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1

Frame 1 Nmax

d [kN]

Column 1 2 3

floor 3 top 5049 15183 3179floor 3 bot 5062 15188 3179floor 2 top 10220 31170 6649floor 2 bot 10228 31173 6649floor 1 top 15399 46772 -floor 1 bot 15403 46773 -

Mmax

d [kNm]

floor 3 top 4334 4553 4814floor 3 bot 2806 3245 3230floor 2 top 2274 3133 6041floor 2 bot 4691 4868 7520floor 1 top 6903 8653 -floor 1 bot 7382 7696 -

Nd [kN] for Mmax

d

floor 3 top 5045 13289 128floor 3 bot 5045 13298 155floor 2 top 10220 28973 792floor 2 bot 5430 26806 1282floor 1 top 11933 43791 -floor 1 bot 9103 41605 -

Mr(Nd) [kNm]

floor 3 top 14778 13355 8586floor 3 bot 14778 13356 8590floor 2 top 12833 15797 8703floor 2 bot 11997 15460 11272floor 1 top 13132 23883 -floor 1 bot 12638 23540 -

Mmax

d Mr(Nd) []

floor 3 top 2933 3409 5607floor 3 bot 1899 2430 3760

floor 2 top 1772 1983 6941floor 2 bot 3910 3149 6671floor 1 top 5257 3623 -floor 1 bot 5841 3269 -

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 1695 409 151floor 3 bot 5934 430 215floor 2 top 405 757 196floor 2 bot 302 288 170floor 1 top 215 260 -floor 1 bot 448 454 -

60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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60 Chapter 5 Linear dynamic analysis

Table 59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2

Frame 2 Nmax

d [kN]

Column 1 2 3

floor 3 top 5080 9161 4694floor 3 bot 5081 9162 4702floor 2 top 10280 17958 9677floor 2 bot 10281 17959 9682floor 1 top 15355 26820 14062floor 1 bot 15355 26820 14064

Mmax

d [kNm]

floor 3 top 4663 6375 4215floor 3 bot 3442 4723 3045floor 2 top 2418 4350 3161floor 2 bot 3083 5195 3722floor 1 top 3046 3981 2841floor 1 bot 3748 5883 5321

Nd [kN] for Mmax

d

floor 3 top 5078 8419 3891floor 3 bot 4841 8417 3875floor 2 top 9834 16461 8758floor 2 bot 10281 16122 8701floor 1 top 14639 25210 13783floor 1 bot 13855 25157 13806

Mr(Nd) [kNm]

floor 3 top 14783 12519 9250floor 3 bot 14745 12518 9248floor 2 top 12766 13849 10110floor 2 bot 12844 13796 12568floor 1 top 13565 20966 13432floor 1 bot 13443 20957 13435

Mmax

d Mr(Nd) []

floor 3 top 3154 5092 4557floor 3 bot 2334 3773 3293floor 2 top 1894 3141 3127floor 2 bot 2400 3766 2961floor 1 top 2245 1899 2115floor 1 bot 2788 2807 3961

Mmax

d Mr(Nd)

MsMr(Ns)(local dyn factor)

floor 3 top 120 193 181floor 3 bot 130 202 184floor 2 top 144 202 192floor 2 bot 141 208 203floor 1 top 164 184 266floor 1 bot 521 228 317

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 63: JRC62663

6 Nonlinear dynamic analysis

The nonlinear dynamic analysis is the most advanced method for predictingthe response of a structure when a load-bearing element is removedquasi-instantaneously The only difference with the linear dynamic analysis is thatinelastic behaviours andor geometric nonlinearities are taken into account

The procedure used for the calculations in SAP 2000 can be summarised asfollows

bull build a FE modelbull define and assign plastic hinges to selected membersbull find the reaction forces from a column to be removed under the self-weight

loadingbull remove this column from the FE model and apply these reactions in its placebull apply these reaction forces again but in the opposite direction using a linear

ramp functionbull perform nonlinear time history analysis with initial conditions and 5 critical

damping

Plastic hinge properties were based on the concrete cross-section size and rebararea and on the stress-strain relationships for concrete and steel [1] In summarythere were 4 types of beam cross-section and 4 types of column cross-section (seeFigure 61)

The stress-strain (σ minus ε) relationship for concrete is assumed parabolic in thefirst phase and constant in the second phase according to the following equation

σc = fc

[

minus

( ε

0002

)2

+ 2ε

0002

]

(61)

where fc is the compressive strength (taken from the test results of Table 21) εis the strain in concrete 0002 is the strain value at which the parabola ends Forstrains between 0002 and 00035 the stress remains constant until failure The plotof the stress-strain relationship defined in SAP 2000 is presented in Figure 62 Forsteel the stress-strain (σminusε) relationship is assumed bilinear (Figure 63) The firstphase is linear elastic with a yield stress of 52456MPa and a modulus of elasticityof 206GPa while the second phase is plastic with a linear hardening and an ultimatestress of 64256MPa (see Table 22)

From the cross-section geometry the material relationships and the normal force(for column only) the moment-curvature relationships are automatically derived in

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 64: JRC62663

62 Chapter 6 Nonlinear dynamic analysis

Figure 61 Cross sections for beams and columns defined in SAP 2000

Figure 62 Stress-strain relation for concrete

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 65: JRC62663

63

Figure 63 Stress-strain relation for steel

SAP 2000 both in an exact and idealised (bilinear) form The different relationshipsare displayed (left-hand side) in Figures 64-67 for beams and in Figures 68-610for columns under different level of normal force (31 kN - 3rd floor 65 kN - 2ndfloor and 97 kN - 1st floor) The strain diagrams at the ultimate concrete strain(00035) are also plotted on the right-hand side of the same figure It should benoted that the presented Figures are for positive moments and when a negativebending moment develops in cross-sections the inverted cross-section is used tocalculate moment-curvature characteristics

In SAP 2000 the plastic hinge behaviour is defined by a piece-wise linearmoment-plastic rotation relationship the characteristics of which are identified fromthe idealised moment-curvature relationship of the section An example is givenin Figure 611 for a beam section point B is defined by the yielding momentand point C by the ultimate moment and the corresponding plastic rotation Thecurve is usually prolonged by a softening and residual branch which has however noimportance in the present study since the plastic hinges never reach their ultimatecapacity For columns this moment-plastic rotation relationship depends on thenormal force and this interaction may be activated in SAP2000

To be able to directly compare the nonlinear results to the linear ones thefollowing nonlinear demand-resistance ratio (DRRnlin) is defined as

DRRnlin =

100timesMmaxMr if no yielding occurred

100(1 +max plastic rotation

ultimate plastic rotation) if yielding occurred

(62)

This nonlinear DRR coincides with the linear DRR in the absence of yielding(DRR lt 100) In the presence of yielding (DRR gt 100) the nonlinearDRR measures the distance to the ultimate plastic rotation (point C of the

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 66: JRC62663

64 Chapter 6 Nonlinear dynamic analysis

Figure 64 Moment-curvature relationship for a beam type 1 cross-section

Figure 65 Moment-curvature relationship for a beam type 2 cross-section

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 67: JRC62663

65

Figure 66 Moment-curvature relationship for a beam type 3 cross-section

Figure 67 Moment-curvature relationship for a beam type 4 cross-section

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 68: JRC62663

66 Chapter 6 Nonlinear dynamic analysis

Figure 68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN

Figure 69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN

67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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67

Figure 610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN

Figure 611 Definition of a plastic hinge for a type 1 beam element

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
  • pc-report2
  • rep2_backpages_ok
Page 70: JRC62663

68 Chapter 6 Nonlinear dynamic analysis

moment-curvature relationship) As for the linear DRR 200 is marking thethreshold not to be exceeded (failure of the section) although this does notnecessarily implies the collapse of the structure

Three plastic hinges are introduced in each beam (left mid and right) and twoin each column (bottom and top) thus resulting in 36 plastic hinges for each frame(Figure 612)

137H1(hcol3)

77H1(hinge_beam)

140H1(hcol3)

141H1(hcol2)

87H1(hinge_beam)

144H1(hcol2)

145H1(hcol1)

97H1(hinge_beam)

148H1(hcol1)

89H1(hinge_beam)

99H1(hinge_beam)

79H1(hinge_beam) 83H1(hinge_beam)

200H1(hcol6)

201H1(hcol5)

93H1(hinge_beam)

204H1(hcol5)

205H1(hcol4)

103H1(hinge_beam)

208H1(hcol4)

85H1(hinge_beam)

95H1(hinge_beam)

105H1(hinge_beam)

86H1(hinge_beam)

96H1(hinge_beam)

106H1(hinge_beam)

152H1(hcol9)

153H1(hcol8)

156H1(hcol8)

157H1(hcol7)

160H1(hcol7)

149H1(hcol9)197H1(hcol6)

82H1(hinge_beam)

92H1(hinge_beam)

102H1(hinge_beam)

Figure 612 Locations of plastic hinges

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 71: JRC62663

One central column removed 69

61 One central column removed

The nonlinear dynamic analysis for one central column removed shows that atat time 0039 s two plastic hinges are activated almost simultaneously in the firstframe one at the top of the right column on the third floor and the other at theright-end of the right beam on the second floor (see Figure 613) Shortly after at0040 s another plastic hinge is activated at the right-end of the right beam on thefirst floor The final configuration of the plastic hinges activated after the suddencolumn removal is shown in Figure 614

Figure 613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal

Figures 615 and 616 show the comparison between the linear and nonlineartime histories of the displacement at node 48 and of the bending moment atthe right-end of the right beam on the first floor respectively In both casesthe sharp change at time 004 s is caused by the formation of the first plastichinges Slightly larger deflections (up to 00315m) are observed during the nonlineardynamic analysis owning by the formation of a few plastic hinges The ratiobetween maximum deflections at point 48 for nonlinear and linear dynamic analysesis 118 thus the global dynamic factor (maximum dynamic displacement dividedby static displacement) is slightly higher that in the linear case (189 instead of16) Conversely the bending moments are much lower that in the nonlinear caseespecially of course where the plastic hinges are activated

Fig 617 presents the results available in SAP 2000 for a beam plastic hinge(element No 96) while Figure 618 shows the case of a column plastic hinge (elementNo 160) These figures include the following information

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 72: JRC62663

70 Chapter 6 Nonlinear dynamic analysis

Figure 614 Final locations of plastic hinges for the central column removal

bull the skeleton path of the plastic hinge (thin black line) including the thresholdpoints (yield ultimate residual) and the associated levels of damage (pink ndash firstyielding blue ndash immediate occupancy cyan ndash life safety and green ndash collapseprevention) The skeleton path is the moment-plastic rotation relationshipwithout normal force

bull the actual path followed (thick black line)bull the current time stepbull the values of the plastic moment and rotation at that current time step

For a beam plastic hinge the actual path follows exactly the skeleton path whilefor a column plastic hinge the actual path usually deviates from the skeletonpath because of the influence of the normal force on the moment-plastic rotationrelationship

In the present case the demands in the plastic hinges are all below their ultimatecapacity In fact according to definition (62) the maximum DRRnlin value is 140in beams and 125 in columns The nonlinear dynamic analysis thus demonstratesthat the structure would have survived a sudden removal of the central column

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 73: JRC62663

One central column removed 71

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

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0Deflection at node 48

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses

0 02 04 06 08 1 12minus250

minus200

minus150

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0Bending moment at rightminusend of element 86

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 616 Bending moment at the right-end of the right beam on the first floorcentral column removed comparison of linear and nonlinear dynamic analyses

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

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0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
  • pc-report2
  • rep2_backpages_ok
Page 74: JRC62663

72 Chapter 6 Nonlinear dynamic analysis

Figure 617 Plastic hinge at the right-end of the right beam on the first floor

Figure 618 Plastic hinge at the top of the right column on the third floor

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 75: JRC62663

One left corner column removed 73

62 One left corner column removed

In the nonlinear analysis of a left corner column removal two hinges are activatedalmost simultaneously at time 0094 s one at the right-end of the left beam onthe first floor the other on the bottom of the left column on the second floor(Figure 619) Figure 620 shows the bending moment distribution at first yieldingThe final distribution of activated plastic hinges is shown in Figure 621 and thecorresponding bending moment diagram in Figure 622

Figure 619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal

The ratio between the maximum deflections at node 25 for nonlinear and lineardynamic analyses is 0117m0091m = 129 leading to a global dynamic factor of212 with respect to the linear static analysis

Figure 623 illustrates the response of the first plastic hinge activated whileFigures 624 and 625 compare nonlinear and linear time histories of displacementat node 25 and bending moment in the first hinge activated respectively Themaximum DRRnlin values are 149 for beams and 134 for columns

This nonlinear dynamic analysis shows that the structure would have surviveda sudden removal of the left corner column Again the total or partial collapsewould not have happened thanks to an appropriate activation of plastic hinges andredistribution of bending moments

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
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Page 76: JRC62663

74 Chapter 6 Nonlinear dynamic analysis

Figure 620 Bending moment distribution at first yielding

Figure 621 Final locations of plastic hinges for the corner column removal

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
  • pc-report2
  • rep2_backpages_ok
Page 77: JRC62663

One left corner column removed 75

Figure 622 Final bending moment distribution

Figure 623 Plastic hinge at the right-end of the left beam on the first floor

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 78: JRC62663

76 Chapter 6 Nonlinear dynamic analysis

0 02 04 06 08 1 12minus012

minus01

minus008

minus006

minus004

minus002

0Deflection at node 25

t [s]

defle

ctio

n [m

]

linear static value

linear dynamicnonlinear dynamic

Figure 624 Vertical deflection at node No 25 left corner column removed

0 02 04 06 08 1 12minus350

minus300

minus250

minus200

minus150

minus100

minus50

0Bending moment at rightminusend of element 82

t [s]

bend

ing

mom

ent [

kNm

]

linear static value

linear dynamicnonlinear dynamic

Figure 625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
  • pc-report2
  • rep2_backpages_ok
Page 79: JRC62663

63 One right corner column removed

When it comes to the case where a right corner column is removed the lineardynamic analysis has shown that the structure remains elastic so the nonlinearanalysis gives exactly the same results as in paragraph 53

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 80: JRC62663

7 Two central columns removed

In all three scenarios considered the structure experienced limited or no damageIn order to assess the robustness of the structure the case of two central columnsremoval has also been studied through linear and nonlinear dynamic analyses

The results have been summarised in Figure 71 where the time history of thedisplacement at node 48 is plotted for the linearnonlinear analysis of onetwocolumn(s) removal It can be seen that no matter whether one or two central columnsare removed from the structure the response does not change drastically which canbe explained by the one-way behaviour of the flat-slab frame In other words eachframe appears to be damaged essentially by the removal of its central column

0 02 04 06 08 1 12minus0035

minus003

minus0025

minus002

minus0015

minus001

minus0005

0Deflection at node 48

t [s]

defle

ctio

n [m

]

1 column linear dynamic1 column nonlinear dynamic2 columns linear dynamic2 columns nonlinear dynamic

Figure 71 Comparison of displacement at node 48 for the cases when one centralcolumn is removed and when two central columns are removed

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 81: JRC62663

8 Conclusions

This report presents the results of an extended study of the flat-slab framebuilding which was analysed and tested quasi-statically at the ELSA Laboratorya few years ago The scope of the previous study was limited to the investigationof the general safety against collapse and thus did not consider a possible abruptremoval of columns as it may take place in the incidence of bomb explosions impactsor other accidental actions

The current investigation includes linear and nonlinear dynamic time historyanalyses using alternate load path methods as presented in the earlier JRC Scientificand Technical Report [6] Three scenarios of column removal have been considereda central column a left corner column and a right corner column

The results of the analyses are summarised in Table 81 This table presents themaximum values of the demand-resistance ratios (in the most critical cross-sections)and the maximum displacements obtained through linear static linear dynamic andnonlinear dynamic analyses The colours highlight the conclusion drawn from eachanalysis in terms of three possible structural states no damage limited damage andextensive damage

The simplest linear static analysis indicates that the structure would exhibitlimited or no damage if the column is removed statically However if the column isremoved dynamically the same analysis (with a factor 2 to account for the dynamicnature of the loading) indicates that the structure would be susceptible to progressivecollapse in two scenarios whereas it would suffer limited damage in the third one

The linear dynamic analysis indicates a slightly more favourable situation thestructure would still be susceptible to progressive collapse for the central columnscenario but not necessarily for the left column scenario as the DRR is slightly below200 Furthermore the structure would remain fully elastic for the right columnscenario The value 2 of the dynamic factor is therefore conservative In fact theactual value of the dynamic factor found in the three scenario ranges from 172 to187 (maximum of the two values reached in beams and columns) Conversely thedynamic factor computed from the displacement ranges from 146 to 16 and thusunderestimates the dynamic effect on the DRR (non conservative estimate)

The linear dynamic analysis has revealed that the local dynamic factor defined ineach section as the ratio between the dynamic and static demand-resistance ratios isunworkable because it does not makes sense for all sections Huge dynamic factorsmay be found in columns for instance but they are usually insignificant because

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
  • pc-report2
  • rep2_backpages_ok
Page 82: JRC62663

80 Chapter 8 Conclusions

they result from the relatively small value of the static force The global dynamicfactor defined from the displacement of the node above the removed element doesnot present such a drawback but remains quite different from the true dynamicfactor computed as the ratio between the dynamicstatic maximum DRR It shouldbe noted that the results may be different for a different structure For examplethe previous studies carried out on another frame structure [6] demonstrated thatthe global dynamic factor (in terms of the displacement at the node above theremoved column under a central column removal) has been larger and equalled 176as compared to 160 obtained in this study

The nonlinear dynamic analysis (taking into account the capability ofredistribution of internal forces) indicates that the progressive collapse of thebuilding would not have happened that is the propagating failure would have beenarrested For both the central and left corner column removals several plastic hingeswould have occurred in the structure yet all of them would have been far below theirultimate capacity (two yellow areas in the summary Table) For the right cornercolumn removal no yielding would have occurred as already foreseen by the lineardynamic analysis

Although the nonlinear dynamic analysis appears to be the most accurate givesthe most information about the behaviour of the structure and leads to a moreeconomic design it should be noted that (1) this type of analysis is also themost time-consuming (plus several computation re-runs) (2) it requires a propermodelling of the reinforced concrete cross-sections (see Figure 61) as well as (3) anappropriate definition and location of plastic hinges in beams and columns

Another finding is that within a given column-removal scenario the most criticalsections (with maximum demand-resistance ratios) may be different in the differenttypes of analysis This can be readily observed following the locations of the reddots in the sketches in Table 81 This lack of consistent pattern is true when passingfrom the linear static to the linear dynamic analyses from the linear static to thenonlinear dynamic analyses as well as from the linear dynamic to the nonlineardynamic analyses This explains why it is so difficult to find a case-independentcorrelation between the different analyses

Finally the second frame in the three column removal scenarios has alwaysexperienced relatively minor distress As proved in Chapter 7 the removal of twocentral columns (one in the first frame and the other in the second frame) causessimilar internal forces and deformations as those of the first frame This meansthat for this particular structure it does not matter whether one or two columnsare removed each frame bridges over the missing column separately Obviously ifone of the frames collapses it will entail the out-of-plane collapse of the other frame

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 83: JRC62663

81

Table 81 Summary table

linear nonlinear

global globalstatic sttimes2 dynamic dyn fact dynamic dyn fact

DRRlin

Beam 325 - - - - -Column 293

Intact

DRRlin DRRlin DRRlin DRRnlin

B 1237 2474 B 2125 172 B 140 113C 1078 2156 C 1598 148 C 125 116

Scenario 1 δ 00167 m

δ

00334 m δ 00268 mδ

160 δ 00315 mδ

189

DRRlin DRRlin DRRlin DRRnlin

B 1326 2652 B 1995 150 B 149 112C 923 1846 C 1728 187 C 134 145

Scenario 2 δ 00552 m δ 01104 m δ 00910 m δ 165 δ 01170 m δ 212

DRRlin DRRlin DRRlin

B 662 1324 B 845 128 no 128C 392 784 C 694 177 yielding 177

Scenario 3 δ 00142 m

δ

00284 m δ 00207 m

δ

146 146

Legend Linear analysis Nonlinear analysis

green no yielding DRRlin lt 100 green no yielding DRRlin lt 100 DRRlin = 100timesMmax

Mr

or 100timesNmax

Nr

yellow limited yielding 100 lt DRRlin lt 200 yellow limited yielding 100 lt DRRnlin lt 200 DRRnlin =

100timesMmaxMr no yielding

100(1 +max plastic rotation

ultimate plastic rot) with yielding

red large yielding DRRlin gt 200 red large yielding DRRnlin gt 200

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
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Page 84: JRC62663

References

[1] ATC Seismic evaluation and retrofit of concrete buildings ATC-40 ReportApplied Technology Council Redwood City California 1996

[2] DoD UFC Guidelines Design of Buildings to Resist Progressive Collapse UnifiedFacilities Criteria (UFC) 4-023-03 Department of Defence (DoD) 2005

[3] EN 1991-1-7 Eurocode 1 - EN 1991-1-7 Actions on structures - Part 1-7General actions - Accidental actions 2006

[4] M Gemelli P Negro A Castellani R Bianchi and M Salandi Experimentalevaluation of the safety against the collapse of buildings Technical ReportI03102 European Commission Joint Research Centre 2003

[5] GSA Guidelines GSA Progressive Collapse Analysis and Design Guidelinesfor New Federal Office Buildings and Major Modernizations Projects GeneralServices Administration (GSA) 2003

[6] S Kokot Literature survey on current methodologies of assessment of buildingrobustness and avoidance of progressive collapse JRC Scientific and TechnicalReports JRC 5598 European Commission Joint Research Centre 2009

[7] P Negro and E Mola Current assessment procedures application to regularand irregular structures compared to experimental results In Third EuropeanWorkshop on the Seismic Behaviour of Irregular and Complex StructuresFlorence September 17-18 2002

[8] NIST Best Practices Best Practices for Reducing the Potential for ProgressiveCollapse in Buildings US National Institute of Standards and Technology(NIST) Washington DC 2007

[9] A Pinto G Verzeletti J Molina H Varum R Pinho and E CoelhoPseudodynamic tests on non-seismic resisting rc frames (bare and selectiveretrofit frames) Technical report European Laboratory for StructuralAssessment 2002

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
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Page 85: JRC62663

List of Figures

21 Front view 6

22 Floor plan 6

23 Elevation and column rebars 7

24 Beam rebars 7

25 Interaction diagram for a type 1 beam 9

26 Interaction diagram for a column with rebars φ14 9

27 Interaction diagram for a column with rebars φ16 10

28 Interaction diagram for a column with rebars φ18 10

29 Interaction diagram for a column with rebars φ20 12

31 Finite element model of the analysed frame in SAP 2000 - element numbers 15

32 Frame model in SAP 2000 - node numbers 16

33 Analysed scenarios of column removal 16

34 Loads on the frame self weight 17

35 Loads on the frame reaction from the actual column at node 48 17

36 Loads on the frame - simulation of the column removal (from SAP 2000) 18

37 Loads on the frame - load case (from SAP 2000) 18

41 Bending moments original structure 20

42 Shear forces original structure 21

43 Axial forces original structure 21

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 86: JRC62663

84 Chapter 8 List of Figures

44 Interaction diagram for a column with rebars φ20 22

45 Bending moments linear static analysis central column removed frame 1 24

46 Axial forces linear static analysis central column removed frame 1 25

47 Bending moments linear static analysis central column removed frame 2 25

48 Axial forces linear static analysis central column removed frame 2 26

49 Bending moments linear static analysis left corner column removed frame 1 29

410 Axial forces linear static analysis left corner column removed frame 1 30

411 Bending moments linear static analysis left corner column removed frame 2 30

412 Axial forces linear static analysis left corner column removed frame 2 31

413 Bending moments linear static analysis right corner column removed frame 1 34

414 Axial forces linear static analysis right corner column removed frame 1 35

415 Bending moments linear static analysis right corner column removed frame 2 35

416 Axial forces linear static analysis right corner column removed frame 2 36

51 Envelope of bending moments linear dynamic analysis central columnremoved frame 1 41

52 Envelope of axial forces linear dynamic analysis central column removedframe 1 41

53 Envelope of bending moments linear dynamic analysis central columnremoved frame 2 42

54 Envelope of axial forces linear dynamic analysis central column removedframe 2 42

55 Vertical deflection at node No 48 central column removed 43

56 Bending moment at the most critical section central column removed 43

57 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 1 47

58 Envelope of axial forces linear dynamic analysis left corner column removedframe 1 48

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 87: JRC62663

85

59 Envelope of bending moments linear dynamic analysis left corner columnremoved frame 2 48

510 Envelope of axial forces linear dynamic analysis left corner column removedframe 2 49

511 Vertical deflection at node No 25 left corner column removed 49

512 Bending moment at most critical section left corner column removed 50

513 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 1 54

514 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 1 55

515 Envelope of bending moments linear dynamic analysis right corner columnremoved frame 2 55

516 Envelope of axial forces linear dynamic analysis right corner columnremoved frame 2 56

517 Vertical deflection at node No 69 right corner column removed 56

518 Bending moment at most critical section right corner column removed 57

61 Cross sections for beams and columns defined in SAP 2000 62

62 Stress-strain relation for concrete 62

63 Stress-strain relation for steel 63

64 Moment-curvature relationship for a beam type 1 cross-section 64

65 Moment-curvature relationship for a beam type 2 cross-section 64

66 Moment-curvature relationship for a beam type 3 cross-section 65

67 Moment-curvature relationship for a beam type 4 cross-section 65

68 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 31 kN 66

69 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 65 kN 66

610 Moment-curvature relationship for a column cross-section (rebars φ14) -under a compression force of 97 kN 67

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

  • rep2_frontpages_ok
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Page 88: JRC62663

86 Chapter 8 List of Figures

611 Definition of a plastic hinge for a type 1 beam element 67

612 Locations of plastic hinges 68

613 The first two plastic hinges activated at approximately 0040 s after thecentral column removal 69

614 Final locations of plastic hinges for the central column removal 70

615 Displacement at node No 48 comparison of linear and nonlinear dynamicanalyses 71

616 Bending moment at the right-end of the right beam on the first floor centralcolumn removed comparison of linear and nonlinear dynamic analyses 71

617 Plastic hinge at the right-end of the right beam on the first floor 72

618 Plastic hinge at the top of the right column on the third floor 72

619 First two plastic hinges activated at approximately 0094 s after the cornercolumn removal 73

620 Bending moment distribution at first yielding 74

621 Final locations of plastic hinges for the corner column removal 74

622 Final bending moment distribution 75

623 Plastic hinge at the right-end of the left beam on the first floor 75

624 Vertical deflection at node No 25 left corner column removed 76

625 Bending moment at right-end of the left beam on the first floor left cornercolumn removed 76

71 Comparison of displacement at node 48 for the cases when one central columnis removed and when two central columns are removed 78

A1 Destruction of the first central column (Phase 1) 90

A2 State of the building at the end of Phase 1 90

A3 Destruction of the other central column (Phase 2) 91

A4 State of the building at the end of Phase 2 91

A5 Demolition of the external columns 92

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 89: JRC62663

A6 Collapse of the building (1) 92

A7 Collapse of the building (2) 93

A8 Collapse of the building (3) 93

A9 Collapse of the building (4) 94

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 90: JRC62663

List of Tables

21 Concrete strength (mean values) 5

22 Steel strength (mean values) 8

23 Resistance of beams 11

24 Resistance of columns 11

41 Bending moments in beams no column removal comparison with resistanceframes 1 and 2 20

42 Axial forces and bending moments in columns no column removalcomparison with resistance frames 1 and 2 23

43 Bending moments in beams central column removed comparison withresistance frame 1 and 2 26

44 Axial forces and bending moments in columns central column removedcomparison with resistance frame 1 27

45 Axial forces and bending moments in columns central column removedcomparison with resistance frame 2 28

46 Bending moments in beams left corner column removed comparison withresistance frame 1 and 2 31

47 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 1 32

48 Axial forces and bending moments in columns left corner column removedcomparison with resistance frame 2 33

49 Bending moments in beams right corner column removed comparison withresistance frame 1 and 2 36

410 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 1 37

411 Axial forces and bending moments in columns right corner column removedcomparison with resistance frame 2 38

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 91: JRC62663

51 Maximum bending moments in beams central column removed comparisonwith resistance frame 1 and 2 44

52 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 1 45

53 Maximum axial forces and bending moments in columns central columnremoved comparison with resistance values frame 2 46

54 Maximum bending moments in beams left corner column removedcomparison with resistance frame 1 and 2 51

55 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 1 52

56 Maximum axial forces and bending moments in columns left corner columnremoved comparison with resistance frame 2 53

57 Maximum bending moments in beams right corner column removedcomparison with resistance frame 1 and 2 58

58 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 1 59

59 Maximum axial forces and bending moments in columns right corner columnremoved comparison with resistance frame 2 60

81 Summary table 81

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 92: JRC62663

A Photos from experimental destroying of

columns

The following photos are taken from [4]

Figure A1 Destruction of the first central column (Phase 1)

Figure A2 State of the building at the end of Phase 1

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 93: JRC62663

91

Figure A3 Destruction of the other central column (Phase 2)

Figure A4 State of the building at the end of Phase 2

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 94: JRC62663

92 Appendix A Photos from experimental destroying of columns

Figure A5 Demolition of the external columns

Figure A6 Collapse of the building (1)

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 95: JRC62663

93

Figure A7 Collapse of the building (2)

Figure A8 Collapse of the building (3)

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 96: JRC62663

94 Appendix A Photos from experimental destroying of columns

Figure A9 Collapse of the building (4)

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 97: JRC62663

European Commission Joint Research Centre ndash Institute for the Protection and Security of the Citizen Title Static and dynamic analysis of a reinforced concrete flat slab frame building for progressive collapse Author(s) Seweryn Kokot Armelle Anthoine Paolo Negro George Solomos Luxembourg Publications Office of the European Union 2010 ndash 94 pp ndash 210 x 297 cm Abstract

The problem of the progressive collapse of a building has been addressed using a reinforced concrete flat slab frame This structure was tested in the past at the ELSA laboratory to evaluate safety margins against collapse Static linear and nonlinear analyses of the building under column removals had first been performed and then in the experiment columns had been successively removed slowly demolished with a crunching machine The experiment showed that the structure survived the demolition of two central columns and also that structural testing against progressive collapse can be very challenging Extending the scope the dynamic nature of the loading has been considered in this report since buildings can be exposed to fast dynamic abnormal events such as explosions or impacts which may destroy abruptly load bearing elements Thus this study tries to answer the question of what would have happened to this building if the columns had been destroyed dynamically For the same structural model dynamic linear and nonlinear analyses have been performed employing the finite element computational framework of the SAP2000 code Alternately three columns a central and two corner ones have been instantaneously removed and the structural response of the frame calculated Maximum values of bending moments and forces at critical sections are reported and compared to those of the static analyses Time histories of deflections of nodes above missing columns are determined and several ratios of response parameters resulting from the different analyses are produced for comparison purposes All approaches predict no mechanism which might lead to progressive collapse even though several hinges are formed Advantages of the use of static or dynamic linear or nonlinear analyses are discussed

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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Page 98: JRC62663

How to obtain EU publications Our priced publications are available from EU Bookshop (httpbookshopeuropaeu) where you can place an order with the sales agent of your choice The Publications Office has a worldwide network of sales agents You can obtain their contact details by sending a fax to (352) 29 29-42758

The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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The mission of the JRC is to provide customer-driven scientific and technical supportfor the conception development implementation and monitoring of EU policies As a service of the European Commission the JRC functions as a reference centre of science and technology for the Union Close to the policy-making process it serves the common interest of the Member States while being independent of special interests whether private or national

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