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Theneedtodesignforrobustness

in

fire

IanBurgess

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Robustnessaworkingdefinition

2

Theabilityofastructuretoavoid

disproportionatecollapsewhen

subjecttoalocalised failure

Hence: Onlystructuralresistancefailureisconsidered

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Whyisitimportantinfire?

3

Multiplelocalised structural

failuresinfire+impact

damage.

Collapseofthewhole

structure,including90%

unaffectedbyfireorimpact.

KeyNISTPostWTCRecommendation

Increasestructuralintegrity

Developdesigntoolsandmodifycodestopreventprogressivecollapse.

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Strain

(%)

0.5 1.0 1.5 2.0

Stress(N/mm2)

0

300

250

200

150

100

50

20C

200C300C

400C

500C

600C

700C

800C

Steelbehaviour athightemperatures

0

0.5

1.0

1.5

2.0

2.5

200 400 600 800

Temperature(C)

ExpansionCoeff /C(x105)

Steel

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Sourcesofdisproportionatecollapse

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Multistoreyconstruction:compositebuildings

6

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Multistoreyconstruction:compositebuildings

7

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Multistoreyconstruction:compositebuildings

8

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Wholestoreyfire

9

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Wholestoreyfire

10

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Beamfailure

12

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Columnbuckling

14

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Columnbuckling

16

Column loadingsredistribute. Mayoverload othercolumns.

Column buckles. Fire spreads.

Possibility of pull-in collapse locally.

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Columnbuckling

18

Further columns buckle. Frame collapse.

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Columnbuckling

19

Further columns buckle. Frame collapse.

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Verticalexternalfirespread

22

Flamescausewindowsto

breakonburningstorey(A)

andthefloorabove(B).

B

E

D

C

A

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Verticalexternalfirespread

24

FIrepropagatesonFloorC,spreadsthroughwindowsto

FloorD.

B

E

D

C

A

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Pullinenhancedbucklingofcolumns

25

B

E

D

C

A

Heatedbeamspullingoncolumn

Greatlyincreasedeffectivelength

Columnitselfweakenedbyheating

Inwardbucklingof

column. Partialor

overallcollapse.

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WTC1:Observedcolumnpullin

26

WTC1exteriorcolumns

bowinginwardacross

mostofthesouthface

betweenfloors95to98at

10.23am.

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WTC1:Numericalmodelling

27

Perimeterwallanalysisfound

an

inward

pull

force

of

27kNateachcolumnatfloors

95to99,starting80minutes

aftertheaircraftimpact,

causedamaximuminward

bowingof790mm.

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C i f i C fl d l SO83 fi

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ConnectionforceinWTCfloormodel: ISO834fire

29

100

50

0

50

100

1500 10 20

Time(min)

Horizontalreaction(kN)

Time(min)

Deflection(mm)

1200

1000

800

600

400

200

0

0 10 20

C ti f i WTC fl d l ISO834 fi

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ConnectionforceinWTCfloormodel: ISO834fire

30

Differentialthermalexpansion

Pushout

Time(min)

Deflection(mm)

1200

1000

800

600

400

200

0

0 10 20

100

50

0

50

100

1500 10 20

Time(min)

Horizontalreaction(kN)

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Connection force in WTC floor model: ISO834 fire

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ConnectionforceinWTCfloormodel: ISO834fire

34

Diagonalstrutsatendsbucklein

cascade

Time(min)

Deflecti

on(mm)

1200

1000

800

600

400

200

0

0 10 20

Pullin

100

50

0

50

100

1500 10 20

Time(min)

Horizon

talreaction(kN)

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Tying forces on connections

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Tyingforcesonconnections

37

Capacityat100%top

chordtemperature

Capacityat80%

topchord

temperature

110

90

70

50

30

1010

30

50

70

90110

0 100 200 300 400 500 600 700 800 900Topchordtemperature(C)

Horizontalre

action(kN)

Reaction

force

TENSION

COMPRESSION

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Connectionfracture

39

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Generation of connection failure

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41

Generationofconnectionfailure

Pushoutforce

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42

Initialextensionofbeams

outstripsshorteningdueto

thermalbowing

Columnpushedout,compressiveforce

onconnections.

CardingtonBeamColumnJointFireTest7

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g

43BeamflangebucklingBeamshearbuckling

Connectiontyingforce

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44

y g

Catenary tensionsinbeams.

Athighdeflectionconnectionforces

reversetotension.

Verticalfirespread

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45

Firespreadsacrossupperstorey.

Effectivelengthofheated

columnincreases.

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Connectionfracture

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47

Compartmentintegritybreached.

Debrisloadingontofloor

maypropagateconnection

failuredownwards.

Connectionfracture

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48

Compartmentintegritybreached.

Debrisloadingontofloor

maypropagateconnection

failuredownwards.

Connectionfracture

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49

Compartmentintegritybreached.

Debrisloadingontofloor

maypropagateconnection

failuredownwards.

Consequencesdiffer:

Columnsarekeyelements failuremaybe

disastrous.

Jointfailuremayinitiatefirespreadandprogressivecollapse.

Axialforceinsteeldownstand ofcomposite

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beam(Ding&Wang)

50

400

200

0

200

400

600

800

0 200 400 600 800 1000 1200

Temperature(C)

AxialForce(kN)

Axialforcein

restrainedbeam

TENSION

COMPRESSION

Strength

Heating

Cooling

Jointfailuresincooling

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51

Onesidedfailures

ofpartialdepthend

plates

Boltshear

infinplate

Nutthreadstrippingin

endplate

FractureincoolingatCardington

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Temperature

A

xialForceinrestrainedbeam

COMPRESSION

Cooling

Onesidedfailureofpartialdepthendplates

duringcoolingphase.

Reducedstiffnessretainsjointintegrity.

Partialfracturemayhappenwhencooling

fromnetcompression

TENSION

Heating

Partialfracture

52

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Whataretheconsequencesofconnection

failureinfire?

WTC7

53

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Sequenceofevents onSeptember11,2001

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55

Time Event

09:59 WTC2Collapse

WTC1NorthTower

08:46 WTC1Impact~92nd floor

Boeing767200,750km/h

WTC2SouthTower

09:03 WTC2Impact~78th floor

Boeing767200,945km/h

10:28 WTC1Collapse;other

buildingimpacts

WTC7

17:20 WTC7 Collapse

SimulatedfireprogressonFloor12

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Firesignitedonupto10floorsatsimilartimes.

Fireslastedcontinuouslyonlyonfloors7

9and11 13.

Observedparticularlyfrom14.30to17.20.

BurningsimultaneouslyinNEcorneron

severalfloorsinmidafternoon.

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