Research Article Vol. 4 (1), 2016. Comparison between ... effect of the section variation in wind...

Pinnacle Engineering amp TechnologyISSN 2360-9508

httpwwwpjpuborgcopy Author(s) 2016 CC Attribution 30 License

Research Article Vol 4 (1) 2016

Comparison between Approximate and Programmed Methods for Structural Systems of TallBuildings

Eng Taghried Isam Mohammed Abdel-magidsup1 Dr Abdallah Khogali Ahmedsup2

sup1School of Civil EngineeringSudan University of Science and Technology Khartoum Sudan

sup2Civil Engineering DepartmentUniversity of Khartoum Khartoum Sudan

Accepted 23 June 2016

ABSTRACT

In this research work a comparison was made between approximate and exact methods of analysis for tall buildings Onerepresentative tall buildings system was chosen for comparison purposes namely the Rigid Frame System The proposedstructure was modelled in a square plan area of 35times35msup2 The models were solely subjected to wind loads Then models wereanalysed using both approximate manual methods and a Finite Element based computer program An Excel sheet was formulatedand developed to carry out the approximate method procedure Results for manual and computerized analysis were comparedfor three different building heights The same structure was subjected to gravity loads in addition to wind loads and was analysedand designed using the Finite Element based computer program The design results were compared for three different heightsObtained results indicated that manual analysis methods are reliable for buildings with heights lower than 25-storeys especiallyfor the higher storeys Developed model would help in quick structural preliminary analysis procedures and preparations oftall buildingsKeywords ETABS Tall buildings preliminary analysis manual analysis computer model design comparison

IntroductionTall buildings have fascinated mankind since beginning ofcivilization Commercial and residential tall structures havebeen a large part of tall building construction The rapidgrowth of population in the capital city of Sudan Khartoumand the pressure on limited land area have influenced tallresidential buildings development High cost of land is a majorreason to drive residential buildings upward Tall commercialbuildings emerged as a result to the need of business activitiesto be as close as possible to each other and to the city centrethereby putting intense pressure on available land spaceIn preliminary stages one can select a system for the structurefrom alternative proposed systems based on theirperformance Deflections and major member forces could bedetermined as well for the chosen system The model that isformed in the preliminary analysis should represent fairlywell the principal modes of action and interaction of the majorstructural elements (Smith et al 1991)The selection of a structural system for buildings is influencedprimarily by intended function architectural considerationsinternal traffic flow nature and magnitude of horizontalloading height aspect ratio influence of material method ofconstruction planned location and to a lesser extent intensityof loading (Taranath 2010)Tall buildings in Sudan majorly fall within 15 storeys up to 20storeys - with only one 29 high rise building (The NTC Tower)Choosing the most cost effective structural system for a highrise building and relying on results of preliminary analysiswith regard to wind loading constituted the main workstudied in this research

Approximations and simplifications adopted in making apreliminary analysis are sometimes huge concerning loadingdistribution plastic hinge formation or when representing acomplex bent as a simple cantileverEven with gross approximations made in simplifying thestructure and affiliated loadings it is generally expected thata preliminary analysis should give results for deflections andmain member forces that are dependably within about 15of the values of the accurate analysis (Smith et al 1991)Studies on frame analysis were not targeted as it is not aneconomical system for super tall buildings Other structuralsystems were studied Kurc and Lulec (2011) investigated theaxial force in columns and walls of a 37-storey model Theirresults indicated that the column and wall axial load mightvary up to 45 depending on the type of analysis and effectsthat were considered Sutjiadi and Charleson (2012)investigated the potential of double-layer space structures tobe applied vertically as a new structural system in super-tallbuildings They concluded that compared with other currenttall structures vertical double-layer space structures arestructurally relatively efficient Kamath and Rao (2012)examined the outrigger system using ETABS software andfound that performance of the outrigger is most efficient fora relative height of the outrigger equal to 05m Zalka (2009)developed closed-form formulae for the static deflection ofsymmetric multi-storey buildings braced by moment-resisting(andor braced) frames (coupled) shear walls and core andthe proposed closed-form solution for the top deflection issimple and reliableFor simple building Ajwad et al (2015) observed that ETABSsoftware gives more economical structure than STAADsoftware and the modelling process is easier

Corresponding Author Eng Taghried Isam Mohammed Abdel-magidsup1School of Civil Engineering Sudan University of Science and Technology Khartoum SudanEmail address tagimamgmailcom

Pinnacle Engineering amp Technology ISSN 2360-9508 Page 2

How to Cite this Article Eng Taghried Isam Mohammed Abdel-magid Dr Abdallah Khogali Ahmed Comparison between Approximate and ProgrammedMethods for Structural Systems of Tall Buildings Pinnacle Engineering amp Technology ISSN 2360-9508 Vol 4 (1) 2016 Article ID pet_243 996-1008 2016

This research addressed a verification of loading percentagevariation and a definition of the level of accuracy ofapproximate methods with increase in height of the structurewhen subjected to a pure wind loadThe research focuses on cost effect (volume of concrete andarea of steel for the structure main elements) upon choosingthe structural system of a defined building Two structuralsystems were compared with height increase for both a framesystem and a dual oneMaterials and MethodsThe case study chosen is a building of a plan area of 35times35msup2divided into 7 panels 5 meters each both ways Buildingsmodels were constructed on ETABS for 15 storeys 20 storeysand 25 storeys to evaluate the best choice of a structural form-with respect to cost efficiency- from two forms rigid frameform and combined rigid frame-shear wall form The modelswere subjected to both gravity and wind loadingsFloor slabs were designed as 20 cm thick slabs and weresubjected to a dead load of 75 kNmsup2 that consists of 45kNmsup2 partitions loads 15 kNmsup2 finishing and 15 kNmsup2for the electrics and air conditioning equipment and to a live

load of 25 kNmsup2 as the model was considered to be an officebuilding (BS 1996)Initial columns and girders sizes were adopted as 300times1000mm and 300times700mm respectively for all floors to minimizethe effect of the section variation in wind load distributionLikewise initial columns orientations were adopted as shownin Fig (3) to minimize the change in stiffness effect on windload distributionWind loads were calculated using principles outlined in theBritish Standard CP3 Chapter 5 Part 2 The basic wind speedwas taken as 45 ms and the building territory was taken as3 The Topography Factor (S1) and the Statistical Factor (S3)were taken as 1 with reference to Table (2) and Figure (2) ofthe code Ground roughness building size and height aboveground Factor (S2) was calculated for the different heightswith reference to Table (3) of the code The design wind speed(Vs) and wind pressure (q) were then calculated Results wereas presented in Table (1) Wind pressure per meter height isreferred to as Q1 in the table F is the wind force per storeyheight and F1 is the wind force per storey per bent

Results from these calculations were assigned to the modelfor analysis

Table (1) Wind loading calculationsStorey Height S2 Vs ms q Nmsup2 Q1 Nm F kN (storey) F1 kN (bent)Ground 3 06 27 44689 156407 2346 335

1 6 0668 3006 55391 1938682 5816 8312ⁿ 9 0722 3249 64708 226479 6794 9713 12 0776 3492 7475 2616236 7849 11214 15 083 3735 85515 2993021 8979 12835 18 0872 3924 94388 3303593 9911 14166 21 0907 4082 102118 3574112 10723 15327 24 0928 4176 106901 3741532 11225 16048 27 0949 4271 111794 3912785 11738 16779 30 097 4365 116796 4087869 12264 1752

10 33 0982 4419 119704 4189638 12569 179611 36 0994 4473 122647 4292658 12878 18412 39 1006 4527 125627 439693 13191 188413 42 1016 4572 128137 4484778 13454 192214 45 1025 4613 130417 4564585 13694 195615 48 1034 4653 132717 4645095 13935 199116 51 1042 4689 134779 4717251 14152 202217 54 1048 4716 136335 4771733 14315 204518 57 1054 4743 137901 4826527 14479 206919 60 106 477 139475 4881635 14645 209220 63 1066 4797 141059 4937055 14811 211621 66 1072 4824 142651 4992788 14978 21422ⁿ 69 1078 4851 144252 5048834 15147 216423 72 1084 4878 145863 5105192 15316 218824 75 109 4905 147482 5161864 15486 221225 78 1096 4932 14911 5218848 7828 1118


Columns and girders were assigned to the selected 35times35msup2plan area Initial frames were adopted parallel to the appliedwind direction The model was analysed using a FiniteElement based computer program (ETABS 972) under solelywind loads Results were then compared with a handcalculated model Two methods for wind loading calculationwere adopted for the hand calculations The Portal Methodand The Cantilever Method (Smith et al 1991) Results werecompared with the computer results for different heightsFigures (5) (6) (8) and (9) show the results of the 20-storeymodel for external column B1 and internal column B2 asreferred to Figure (1) Figure (7) shows comparison of resultsfor bending moment of the girder at the top middle and lowlevels Perpendicular frames were then added to account forthe change in wind directionPortal MethodThe shear force in each storey was allocated to the columnsin proportion to the aisle width they support as indicated inequation 1VCij = VjtimesaijƩLGij helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(1)

WhereVCij = Shear force in column i at storey level jVj = Shear force at storey level jaij = Aisle width supported by column iLGij = Length of girder i at storey level j

From the columns shears and storey height maximummoments at the columns were found as presented in equation2MCij = VCijtimesHj2 helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(2)

Where

MCij = Bending moment for column i at storey level jHj = Height of storey j

Girder-end moments were found from the equilibrium of thejoints and girder shear were found by dividing the girder-endmoment by half the span (Smith et al 1991)Cantilever MethodThe second moment of the column areas about their centroidwas found using equation 3I = Ʃaicisup2 helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(3)

WhereI = Second moment of column areasAi = Area of column iCi = Distance of column i from the centroid

Column axial forces in each storey were found using equation4Fi = MciAiI helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(4)

WhereM = The storey moment resulting from wind loadGirder shears were found from the vertical equilibriumgirder-end moments were then found Column maximummoments were found from the equilibrium of joints thencolumn shear were evaluated (Smith et al 1991)Results and DiscussionFor manual analysis purpose Shear forces and bendingmoments due to wind loading for each storey were calculatedResults for the 20-storey model are shown in Figures (1) and(2) The selected area is shown in Figure (3) a section of whichis shown in Figure (4) for 20-storey model

Figure (1) Wind Bending Movement



Figure (3) Rigid Frame Model Plan Figure (4) Rigid Frame Model Section at Grid B



Figure (5) 20-Storey Model Column (B1) Wind Load Moments






Comparison between manual calculations and ETABS modelfor column bending moments are shown in Figures (5) and(6) Two columns are chosen for the comparison internalcolumn and external column Columns are defined withreference to Figure (3)Taking the FEM as the reference manual results for bendingmoment are shown to be giving approximately accurateresults and can be used to estimate the bending moments oncolumns for top storeys The results at the bottom storey areshown to vary from the FEM results Hence the manual

methods seem to under-estimate the bending momentsapplied to columns at the bottom storeysThe Cantilever method appears to give results approximatelysimilar to the program results Though it shows little negativedifference in the inner columns the Portal method appears toover-estimate bending moments in the outer columnsAs the building height increases the Cantilever methodappears to be giving more accurate results and hence can beused to estimate the bending moments for tall buildings forpreliminary design

Figure (7) beam bending Moments



Figure (7) Cont beam bending Moments





Pinnacle Engineering amp Technology ISSN 2360-9508 0

Comparison between manual calculations and ETABS modelfor girder bending moments are shown in Figure (7) Onegirder (girder at grid A Figure (3)) was chosen for thecomparison from the 20th storey down to the ground levelstoreyTaking the FEM as the reference the girders moments aremore accurately estimated by the Portal Method at the middlestoreys The method tends to overestimate the girdermoments at both end bays and under-estimates them at themiddle bays for the top storeys

The Cantilever Method appears to slightly over-estimate thebending moments on the girders at all levelsThe actual bending moment diagram appears to be completelydifferent from the diagrams estimated by both manualmethods for the top two storeys This was justified by theshear deflection mode of the rigid frame which tends toreverse the girder moments at the top storeys and this wasnttaken into account in the approximate methods



Comparison between manual calculations and ETABS modelfor column shear forces are shown diagrammatically inFigures (8) and (9) The two columns compared for momentsare chosen for shear comparison for the 20-storey modelColumns are defined with reference to Figure (3)The over-all trend of the shear force change over the heightseems to be well demonstrated by both manual methodsThe Portal Method gives identical shear forces at ground leveland at base which is because the applied wind load was notgiven a value at the ground level considering it as the zerolevel of wind loading The portal method calculates the shearforce on each column by allocating the storey shear to thecolumns in proportion to their aisle widths as the storey sheardid not increase at the ground level the shear force on columnsshould stay the same as shown by the resultsThe Cantilever Method also gives identical shear forces at theground level and at the base which could be explained by thefact that the method transforms the wind moments into shearforces at the girder then to girder moments subsequentlycolumn moments and finally column shears This implies thatthe method depends on girders to receive wind shear forcesThis may lead to the idea that the slight increase in wind shearforce at the base is taken by the girder not by the columnsThis does not apply to the 25 storey model as it may be shownthat there is a remarkable change in shear force at the basefor both inner and outer columns this may be explained bythe fact that he increase in wind moment is significantThe significant drawback in shear force at the bottom storeysthat is shown by ETABS results can be justified by the racking

behaviour of the frame This generates moments resulting inshear forces that applies to the opposite direction of thosecaused by the wind loads resulting in a diminution in shearforces at that level This effect is more recognizable at theouter columns which act as the outer fibres of the verticalcantileverIn an over-all view the Cantilever method givesapproximately accurate results slightly over-estimated forthe outer columns for all heights and slightly under-estimatedat bottom storeys for inner columns for all heights The Portalmethod gives over-estimated shear force results in columnsCantilever method can be a useful tool for preliminary analysisDesign of Structural MembersFloor slabs were meshed 1 meter each way Framesperpendicular to the assumed direction of wind loadingapplication were added to the model and column directionswere changed to account for change in wind direction Thenew column arrangement is shown in Figure (10) The windload direction is shown by the arrows Only one direction wasconsidered in the analysis and design procedureDesign procedure consisted of several trials where thesections of the columns were changed in every trial step toobtain optimum design sections Hence the best combination-with respect to quantities-of concrete sectionsreinforcementsteel was chosen The design procedure focused on designingthe columns because the floor framing does not change muchwith height therefore it remains constant for all models

Figure (10) Rigid Frame Model Plan (Modified Frames)



Figure (11) shows the change in reinforcing steel and columnsection with height for column A-1 as referred to in Figure(10) The X-axis indicates the reinforcement area in squaredmillimetres the Y-axis indicates the storey number and thebubble size indicates the column section Each column sectionused in the design was assigned to a number as followsSection 50times25cmsup2 assigned to number 1Section 60times25cmsup2 assigned to number 2Section 70times25cmsup2 assigned to number 3

Section 80times25cmsup2 assigned to number 4Section 90times25cmsup2 assigned to number 5Section 100times25cmsup2 assigned to number 6Section 110times25cmsup2 assigned to number 7Section 100times30cmsup2 assigned to number 8Section 110times30cmsup2 assigned to number 9Section 120times30cmsup2 assigned to number 10Section 120times35cmsup2 assigned to number 11Section 130times30cmsup2 assigned to number 12Section 130times40cmsup2 assigned to number 13

Figure (11) Column A-1 Comparison of Quantities



Comparison charts show that as height increases the requiredreinforcement increases at top levels for outer columns Thismay be attributed to the effect of wind overturning momentwhich is not suppressed by gravity loads at high levels Therequired reinforcement and concrete section increases atbottom levels for inner columns due to the increase in gravityloadingThe 25-storey building shows major increase in materialquantities when compared with 20-storey building The15-storey building shows minimal material quantities whichmakes the rigid frame system a good candidate for storeys upto 20ConclusionsFrom the research work undertaken herein the followingconclusions emerged1 Manual methods of analysis for rigid frames were found

to give reliable results for the preliminary stages of thedesign process

2 The final design for rigid frames should be madeconsulting the finite element method FEM results ofanalysis This is because the manual methods tend tounder-estimate forces on structural members forbuildings higher than 20 storeys

3 The rigidity and coherence of the rigid frame render it avery suitable system at the proposed building with heightlower than 25-storeys This is due the tendency of usedmaterials to increase when height exceeds namedaforementioned value

References

1 Ajwad A Qureshi L A Ashraf M S Mannan A Zaib N andKalsoom S (2015) Comparison of computer aided analysis anddesign of multi-storey hospital building using ETABS 95 ANDSTAAD PRO2005 ProQuest Materials Science Journals AsiaNetPakistan (Pvt) Ltd Volume27 Issue5

2 American Society of Civil Engineers (2013) Minimum DesignLoads for Buildings and Other Structures ASCESEI 7-10 DOI1010619780784412916

3 BS (1996) British Standard 6399-1 Loading for Buildings- Part1 Code of Practice for Dead and Imposed Loads

4 BS (1972) British Standard CP3 Code of Basic Data for theDesign of Buildings Chapter V Loading Part 2 Wind Loads

5 ETABS (2015) Users Guide Integrated Building DesignSoftware Computers amp Structures

6 Kamath K Divya N and Rao A U (2012) A Study on Static andDynamic Behavior of Outrigger Structural System for TallBuildings Bonfring International Journal of IndustrialEngineering and Management Science 12 Volume 2 Issue 4DOI 109756BIJIEMS1655

7 Kurc O and Lulec A (2013) A comparative study on differentanalysis approaches for estimating the axial loads on columnsand structural walls at tall buildings The Structural Design ofTall and Special Buildings Volume 22 Issue 6 DOI101002tal699

8 Smith B S and Coull A (1991) Tall Building StructuresAnalysis and Design Wiley-interscience Canada

9 Sutjiadi Y and Andrew W (2014) Structural design andanalysis of vertical double-layer space structures in super-tallbuildings Journal The structural design of tall and specialbuildings Volume 23 Issue 7 Page 512-525 DOI101002tal1057

10 Taranath B S Reinforced Concrete Design of Tall BuildingsCRC Press Boca Raton Fl 2010

11 Yuan Yong Cui Junzhi Mang Herbert A (2009) ComputationalStructural Engineering Proceedings of the InternationalSymposium on Computational Structural Engineering held inShanghai China June 22-24 2009 Springer Netherlands DOI101007978-90-481-2822-8

12 Zalka K A (2009) A simple method for the deflection analysisof tall wall-frame building structures under horizontal load TheStructural Design of Tall and Special Buildings Volume18Issue3 DOI 101002tal410


Pinnacle Engineering amp Technology ISSN 2360-9508


This research addressed a verification of loading percentagevariation and a definition of the level of accuracy ofapproximate methods with increase in height of the structurewhen subjected to a pure wind loadThe research focuses on cost effect (volume of concrete andarea of steel for the structure main elements) upon choosingthe structural system of a defined building Two structuralsystems were compared with height increase for both a framesystem and a dual oneMaterials and MethodsThe case study chosen is a building of a plan area of 35times35msup2divided into 7 panels 5 meters each both ways Buildingsmodels were constructed on ETABS for 15 storeys 20 storeysand 25 storeys to evaluate the best choice of a structural form-with respect to cost efficiency- from two forms rigid frameform and combined rigid frame-shear wall form The modelswere subjected to both gravity and wind loadingsFloor slabs were designed as 20 cm thick slabs and weresubjected to a dead load of 75 kNmsup2 that consists of 45kNmsup2 partitions loads 15 kNmsup2 finishing and 15 kNmsup2for the electrics and air conditioning equipment and to a live

load of 25 kNmsup2 as the model was considered to be an officebuilding (BS 1996)Initial columns and girders sizes were adopted as 300times1000mm and 300times700mm respectively for all floors to minimizethe effect of the section variation in wind load distributionLikewise initial columns orientations were adopted as shownin Fig (3) to minimize the change in stiffness effect on windload distributionWind loads were calculated using principles outlined in theBritish Standard CP3 Chapter 5 Part 2 The basic wind speedwas taken as 45 ms and the building territory was taken as3 The Topography Factor (S1) and the Statistical Factor (S3)were taken as 1 with reference to Table (2) and Figure (2) ofthe code Ground roughness building size and height aboveground Factor (S2) was calculated for the different heightswith reference to Table (3) of the code The design wind speed(Vs) and wind pressure (q) were then calculated Results wereas presented in Table (1) Wind pressure per meter height isreferred to as Q1 in the table F is the wind force per storeyheight and F1 is the wind force per storey per bent

Results from these calculations were assigned to the modelfor analysis

Table (1) Wind loading calculationsStorey Height S2 Vs ms q Nmsup2 Q1 Nm F kN (storey) F1 kN (bent)Ground 3 06 27 44689 156407 2346 335

1 6 0668 3006 55391 1938682 5816 8312ⁿ 9 0722 3249 64708 226479 6794 9713 12 0776 3492 7475 2616236 7849 11214 15 083 3735 85515 2993021 8979 12835 18 0872 3924 94388 3303593 9911 14166 21 0907 4082 102118 3574112 10723 15327 24 0928 4176 106901 3741532 11225 16048 27 0949 4271 111794 3912785 11738 16779 30 097 4365 116796 4087869 12264 1752

10 33 0982 4419 119704 4189638 12569 179611 36 0994 4473 122647 4292658 12878 18412 39 1006 4527 125627 439693 13191 188413 42 1016 4572 128137 4484778 13454 192214 45 1025 4613 130417 4564585 13694 195615 48 1034 4653 132717 4645095 13935 199116 51 1042 4689 134779 4717251 14152 202217 54 1048 4716 136335 4771733 14315 204518 57 1054 4743 137901 4826527 14479 206919 60 106 477 139475 4881635 14645 209220 63 1066 4797 141059 4937055 14811 211621 66 1072 4824 142651 4992788 14978 21422ⁿ 69 1078 4851 144252 5048834 15147 216423 72 1084 4878 145863 5105192 15316 218824 75 109 4905 147482 5161864 15486 221225 78 1096 4932 14911 5218848 7828 1118





Where















































References


















Where















































References





















































References














Research Article Vol. 4 (1), 2016. Comparison between ... effect of the section variation in wind...

Documents

Transcript of Research Article Vol. 4 (1), 2016. Comparison between ... effect of the section variation in wind...