Backstay Effect Due to Podium Structure Interaction
Riddesh Naik1 , Roshni John2 , Dinesh Joshi31ME Student, Dept. of Civil Engineering, Saraswati College of Engineering, Kharghar, Maharashtra, India
2Associate Professor, Dept. of Civil Engineering, Saraswati College of Engineering, Kharghar, Maharashtra, India
3Associate Professor, Dept. of Civil Engineering, Saraswati College of Engineering, Kharghar, Maharashtra, India
Abstract - Marvels of the modern world cannot be imagined without the involvement of enormous superstructures which form the very core of visual attraction for metropolitan cities. Modernization and urbanization have led to migration to a vast extend, causing scarcity of land. Economical and efficient use of land is the need of the hour as horizontal expansion of cities is a complicated task. So, to overcome this scenario the vertical expansion of cities in the form of Highrise structures is to be undertaken. To enhance the efficiency of a particular land, developers and architects have come forward to include below storey podium structure to Highrise buildings. These structures prove to be more efficient as they can be used for various commercial purpose. Focus of current work is to study the effects of these augmented portions of structure in the form of podiums, on the structural aspect of the entire structure. A 40 Storey RCC structure is been considered situated in the IVth seismic zone of India. Various effects of inclusion of podium are studied by comparing structures with and without the inclusion of below storey podium. Effect of the increased stiffness due to podium is studied by considering various structural parameters like Lateral Displacement, Storey Shear, Storey Drift, Time Period. Studies regarding Backstay Effect due to below storey podium structure are made and its effects are observed at the podium structure interaction level. Sensitivity analysis is been carried out as per IS 16700 to study effect of crack formation.
Key Words: Highrise buildings, Backstay Effect, Storey Shear, Storey Drift, Time Period, Sensitivity Analysis, Below Storey Podium, Podium Structure Interaction.
1.INTRODUCTION
Increased job availability in major cities as a result of urbanisation has indirectly increased migration to these areas.Thepopulationofthesecitiesgrowsasaresult.The effects of population growth are directly related to the liveable area of a specific city. Population growth, urbanization, and the need for diverse infrastructure all contributed to the scarcity of adequate land for development.Becausethereisalackofland,itisdifficultto expand cities horizontally; therefore, tall buildings are constructedasasolutiontothisproblem.Consequently,in order to gain leverage and satisfy the need for larger commercial space close to the road level, architects and developers have come up with a plan to provide a below storeypodiumtoanexistingstructure.
Atthebaseofmedium-riseandtallstructures,podiumsare expanded floor surfaces. Metropolitan areas with low to moderate seismic activity prefer this type of construction since a structure with this configuration can support a varietyoffunctionalities. (i.e.,retail spaceinthepodium's lower levels and housing or office space in the tower). Momentresistantframesandshearwallsmakeupthelateral load-resistingsystemforsuchbuildingconstructions.Many tallstructureshaveanarrangementinwhichthelowerfew storeyshaveawiderfloorplanthanthetowerabove.Any lowercomponentofatallstructurewithawiderfloorplan andmuchhigherseismicforceresistancethantheportion aboveitcanbeviewedasa podium structure.Thissortof architecture is typical in multi-story buildings where the lower section of the storeys is commonly utilised for commercialspaces,retailstores,parkinglots,etc.
1.1 Setback/Backstay Effect
Incomparisontotheabovestructure,thesurfaceareaofthe below storeypodium structureis significantly larger. This largershiftinstiffnessoccursacrosstheentirestructuredue tothepodiumstructure'sdifferentdimensions.Asaresult, thepodiumstructure'srigidityissubstantiallyhigherthan that of the tower structure above. As a result, the podium structurehelpsinresistinglateralloadsthatactontheentire structure.Attheselevels,lateralloadresistancewithinthe podiumlevelwithaguaranteedloadtransferpaththrough thefloordiaphragms,aidsintheoverallstructure'sabilityto withstand lateral stresses. This aspect of the structure's resistancetolateralloadsisknownastheBackstayeffector shearreversalbecauseshearforcesinthestructuretendto changedirectionatthepodiumstructureinteractionlevel.To equilibratethe lateral forces and moment of a tower projectingaboveapodiumstructure,asetoflateralforces aredevelopedwithinthepodiumstructure,whichhelpsto resist the external lateral forces, this is termed as the backstayeffect.
1.2 Sensitivity Analysis
Whencracksareformedinamemberthemomentcarrying capacityorstiffnessofthemembertendstoreduce.Exact extendofthesecrackscannotbedetermined.Onceacrackis formed in a member, its stiffness reduces. Thus, different stiffness modifiers are applied to different structural elements in order to account the effect of cracking on stiffness of section and there by on the behaviour and
analysisresultsofabuildingstructure.SensitivityAnalysisis a procedure to assess the behaviour of a building under different scenarios by gradually changing the stiffness propertiesofitsstructuralelements.AccordingtoIS16700 twosetsofsensitivityanalysisshouldbecarriedoutusing upperboundandlowerboundstiffnessmodifiersprovided intableno.7,clause8.1.3.2.1asapartofcollapseprevention evaluation to study the effect of these modifiers on the behaviourofthestructure.Upperboundandlowerbound stiffness modifiers should be applied in the below storey podiumstructurealongwithcracksectionmodifiers tobe appliedintheabovetower.
2. METHODOLOGY
2.1 Problem Statement
To study the effect of podium structure interaction on the structuralparametersofthebuilding,multiple40storeyRCC structures with and without a below storey podium and having a shear wall core are modelled and analysed. ConsideringthemtobeinIVth seismiczoneandanalysingit withresponsespectrumanalysisforseismicanalysis.
2.2 Objectives
Analysea40-storeystructurewithandwithoutabelow storeypodiumstructurebyresponsespectrumanalysis.
Study the effect of podium structure interaction by comparing the results with a normal structure without theinclusionofapodiuminit.
Studyeffectofbackstayonstructuralparametersofthe structure like lateral displacement, base shear, time periodandstoreydrift.
Studytheeffectofupperboundstiffnessmodifiersand lowerboundstiffnessmodifiersonthemodels.
Study the effect of height of podium on the results by consideringdifferentheightofpodiumsattachedtothe mainstructure.
Study the effect of surface area of the below podium structureontheresultsbyconsideringdifferentsurface areaofpodiumsattachedtothemainstructure
3. MODELLING
3.1 Analysis
In the presented study, Dynamic analysis is performed undertheguidelinesofIS16700(2017),IS1893:Part1 (2016), IS 456 (2000), IS 13920 (2016), IS 875: Part 1 (2015),IS875:Part2(1987)andIS875:Part3(2015).
40StoreySMRFRCCstructureswithheightof120mare modelled with and without below storey podium structures.
Responsespectrumanalysisisuseforseismicanalysis
SensitivityanalysisiscarriedoutasperIS16700withthe consideration of upper bound stiffness modifiers and lowerboundstiffnessmodifiers.
Rigiddiaphragmsareusedforallfloors.
Dimensionsofeverystructuralmemberiskeptthesame throughoutallthemodelsforthecomparativestudy.
SlabsandShearWallsaremodelledasthinshells.
Mass source consideration for loads is 1DL, 0.5LL and 1SIDL as live load to be considered is greater than 3KN/m2
A total of 13 models are considered for analysis and comparativestudy.
Results are displayed in terms of lateral displacement, baseshear,timeperiod,storeydriftandshearreversed.
Effect of backstay on various structural parameters is beenstudiedduetothepodiumstructureinteraction.
For sensitivity analysis upper bound and lower bound stiffness modifiers are applied as per IS 16700, clause 8.1.3.2.1inthebelowstoreypodiumstructureandcrack sectionmodifiersareappliedinthetowerabove.
3.2 Input Parameters
Table -1: InputData
A Building Structure
1 Heightof structure 120m
2 No.ofStoreys 40
3 PlanSize
a Conventional Structure 42x35m2 (6x5baystructureof7m c/c,height120m)
b 40Storeyswith 10levelpodium 98x91m2(14x13bayof7mc/c,6x5 baystructureabovepodium)
c 40 Soreys with 6levelpodium 98x91m2(14x13bayof7mc/c,6x5 baystructureabovepodium)
d 40Storeyswith 10levelpodium 70x63m2 (10x9bayof7mc/c,6x5 baystructureabovepodium)
e 40Storeyswith 6levelpodium 70x63m2 (10x9bayof7mc/c,6x5 baystructureabovepodium)
4 Slab Thickness (M50) 150mm
5 Beams(M40) 700x350mm
6 Columns(M50) 1st to10th storey-1100x1100mm 11th to20th storey1000x1000mm
21st to40th storey-900x900mm
7 ShearWalls (M50) 300mm
B Loading
1 LiveLoad 4KN/m2
2 FloorFinishes 1.5KN/m2
3 WallLoad Considering230mmwallsmade upfromengineeringbricks.
Thickness=230mmig
WeightDensityofBrickwork =21.20KN/m3
WallLoad=(3-0.7)x0.23x21.20 =11.215KN/m
ParapetLoad=1.2x0.23x21.20 =5.85KN/m
C Seismic Loading (IS 1893 Part I)
1 SeismicZone IV(IS1893PartI)0.24
2 SoilType II(Medium)
3 Importance Factor 1
4 Response Reduction Factor
5(SMRFStructure)
D Wind Load (IS 875 Part 3)
1 WindSpeed 50m/s
2 Terrain Category 2
E Material Properties
1 GradeOf Concrete Slabs-M50
Columns-M50
ShearWalls-M50
Beams-M40
2 GradeOfSteel Fe415 Fe500
3 WeightDensity 25KN/m3
Table – 3: ModelConfiguration
Sr. No. ModelName ModelConfiguration
A Base Models
1 Conventional Structure(CS) 40Storeystructurewithoutbelow storeyPodium.
2 10SP 40storeystructurewith podium(98x91m2)upto10th storey.
3 6SP 40storeystructurewith podium(98x91m2)upto6th storey.
4 10SSP 40storeystructurewith podium(70x63m2)upto10th storey.
5 6SSP 40storeystructurewith podium(70x63m2)upto6th storey.
B Models with Upper Bound Stiffness Modifier
6 10SPUM 40storeystructurewith podium(98x91m2)upto10th storeywithupperboundstiffness modifiers
7 6SPUM 40storeystructurewith podium(98x91m2)upto6th storey withupperboundstiffness modifiers.
8 10SSPUM 40storeystructurewith podium(70x63m2)upto10th storeywithupperboundstiffness modifiers
9 6SSPUM 40storeystructurewith podium(70x63m2)upto6th storey withupperboundstiffness modifiers.
C Models with Lower Bound Stiffness Modifier
10 10SPLM 40storeystructurewith podium(98x91m2)upto10th storeywithlowerboundstiffness modifiers.
11 6SPLM 40storeystructurewith podium(98x91m2)upto6th storey withlowerboundstiffness modifiers.
12 10SSPLM 40storeystructurewith podium(70x63m2)upto10th storeywithlowerboundstiffness modifiers.
13 6SSPLM 40storeystructurewith podium(70x63m2)upto6th storey withlowerboundstiffness modifiers.
4. Results and Discussion
4.1 Lateral Displacement
Chart-1,Chart-2,Chart–3,Chart–4,Chart–5andChart-6 represent the values for lateral displacement in X and Y Directionforresponsespectrumanalysis.
Chart -1: LateralDisplacementinXDirectionforbase modelsCS,10SP,6SP,10SSPand6SSP.
Chart -2: LateralDisplacementinXDirectionformodels 10SP,10SSP,10SPUM,10SSPUM,10SPLMand10SSPLM.
Chart -3: LateralDisplacementinXDirectionformodels 6SP,6SSP,6SPUM,6SSPUM,6SPLMand6SSPLM.
Fromchart-1andchart-4,itisbeenobservedthatthevalues oflateraldisplacementformodelswithbelowstoreypodium arefarlesserthanbasicconventionalstructuredepictingan increaseingeneralstiffnessofthesemodelsascomparedto basicstructure.
Themaximumvaluestendtodecreasewiththeincreasein the height of the Podium structure in the models. The maximumvaluesforlateraldisplacement inXdirectionfor 10SP and 6SP models are reduced to 58% and 72% respectively when comparedwith the basic conventional structurewithoutbelowStoreyPodium.InYdirection the samevaluesarereducedto41%and59%respectively.
Models with reduce surface area of the podium structure were observed to be stiffer than other models with same podiumheightsasthevaluesforlateraldisplacementwere lesser as compared to other models. Maximum values for lateraldisplacementinXdirectionfor10SSPand6SSPare reduced to 46% and 64% respectively when compared to thebasemodelwithoutbelowstoreypodium.SimilarlyinY directionvaluesarereducedto33%and52%respectively.
For sensitivity analysis, it can be observed from chart-2, chart-3,chart-5andchart-6thatmodelswithupperbound stiffnessmodifierswerealotstifferthanmodelswithlower boundstiffnessmodifiers,yettheywereobservedtobemore flexibleascomparedtomodelswithfactoredloadproperty modifiers.
For10SPmodelthevalueof10SPUMisgreaterby8%and for10SPLMthemaximumvaluegoesuptoapproximately 134%inXDirection.WhileconsideringforYdirectionthe valueof10SPUMisgreaterby7%andfor10SPLMthevalue isgreaterby118%.
For6SPmodelthemaximumvalueforlateraldisplacement inXdirectionfor6SPUMisgreaterby5%andfor6SPLMthe values rise up approximately by 61%. In Y direction the maximumvalueisgreaterby2%andfor6SPLMthevalueis greaterby35%ascomparedto6SP.
For model 10SSP the maximum value for lateral displacementinXdirectionfor10SSPUMisgreaterby7% and for 10SSPLM the value is greater by approximately 125%. When considered in Y direction the value for 10SSPUMisgreaterby7%andfor10SSPLMthevaluegoes upto89%respectivelyascomparedtothebasemodelwith factoredloadpropertymodifiers.
For6SSPmodelthemaximumvalueforlateraldisplacement inXdirection,for6SSPUMisgreaterby5%andfor6SSPLM thevaluesaregreaterby49%.InYdirectionthevaluefor 6SSPUM is greater by 1% and for 6SSPLM the value increasesbyapproximately25%.
4.2 Storey Shear
Chart-7,Chart-8,Chart-9,Chart-10,Chart-11,Chart-12 representthevaluesforstoreyshearinXandYdirection forresponsespectrumanalysis.
Fromabovecharts,itcanbeobservedthatstructuralmodels withtheinclusionofbelowstoreypodiumstructuretendto havehighervaluesofbaseshearas comparedtothebasic conventionalmodelwithoutbelowstoreypodiumstructure astheytendtohavehigherseismicweightduetoincreased structural elements in the podium. Among the presented structures,model10SPhasthehighestvalueforbaseshear
which is 49243KN in X direction and 47505KN in Y direction.
Maximumvaluesforstoreyshearwereobservedatthebase ofeverymodel.ThevaluesofbaseshearinXdirectionforthe models10SPand6SPwereobservedtobegreaterby202% and 142% when compared to the values of the basic conventional structure. For Y direction the values were greater by 219% and 156% for the respected models as comparedtothebasemodel.
Values for base shear decreases with the decrease in the surface area of the podium structure. Models withreduce surfaceareaofthepodiumstructureshowcasedlesservalues whencomparedtoothermodelswiththesameheightofthe podiumwithbiggersurfacearea.
MaximumvaluesofbaseshearinXdirectionfor10SSPand 6SSPwereobservedtobegreaterby36%and 21%when compared to the values of the base structure CS. For Y directionthevaluesweregreaterby36%and21%forthe respectedmodelsascomparedtothebasestructuralmodel CS
Maximum values of base shear for the models with upper and lower bound stiffness modifiers were similar to their respective base models with same podium configuration withfactoredloadpropertymodifiers.
4.3 Storey Drift
Chart -13, Chart -14, Chart -15, Chart -16, Chart -17 and Chart -18 represent the values for Storey Drift in X and Y Directionforresponsespectrumanalysis.
Chart -14: StoreyDriftinXDirectionformodels10SP, 10SSP,10SPUM,10SSPUM,10SPLMand10SSPLM.
Chart -15: StoreyDriftinXDirectionformodels6SP, 6SSP,6SPUM,6SSPUM,6SPLMand6SSPLM.
Chart -16: StoreyDriftinYDirectionformodelsCS,10SP, 6SP,10SSP,6SSP.
From chart-13 and chart-16 it can be observed that the valuesfor maximumstoreydrifttendtodecreasewiththe inclusion of the below storey podium structure indicating theincreaseinstiffnessofthestructure.
Thevaluesofstoreydrifttendtodecreasewithincreaseof height of below storey podium structure. The maximum valuesofstoreydrift forthe Structures10SP and 6SPinX direction are found to be reduced to 77% and 79% respectivelyofthevaluesofthebasicconventionalmodelCS, without below storey podium. For Y direction the same values are reduced to 53% and 67% for the respective modelswhencomparedtothebasemodel’svalues.
Maximumvaluesforstoreydriftareobservedtobereducing evenmoreforstructuralmodelswithreducepodiumsurface area. Models with reduced surface area of the podium structureshowcasedlesservalueswhencomparedtoother modelswithsame height of podium structure with bigger surfacearea.
Maximum values of storey drift for the structural models 10SSPand6SSPinXdirectionarefoundtobereducingto 56%and70%respectivelyofvalueofthebaseconventional
structuralmodelCS.SimilarlyforYdirectionthevaluesare reducedto41%and58%respectivelyofthevaluesofbase model.
Forsensitivityanalysis, it can be observed from chart-14, chart-15, chart-17 and chart-18 that models with upper boundstiffnessmodifiertendtoshowslightlyhighervalues ofstoreydriftwhencomparedtomodelswithcracksection propertymodifiersbutthevalueswerefarlessascompared tomodelswithlowerboundstiffnessmodifiers.Maximum values for storey drift are observed in models with lower boundstiffnessmodifiers.
For10SPmodelthevalueof10SPUMisgreaterby7%and for10SPLMthemaximumvaluegoesuptoapproximately 88% in X Direction. While considering for Y direction the valueof10SPUMisgreaterby5%andfor10SPLMthevalue isgreaterby91%.
For6SPmodelthevalueof6SPUMisgreaterby3%andfor 6SPLM the maximum value goes up to approximately 47% in X Direction. While considering for Y direction thevalueof6SPUMisgreaterby2%andfor6SPLMthevalue isgreaterby29%.
For10SSPmodelthevalueof10SSPUMisgreaterby5%and for10SSPLMthemaximumvaluegoesuptoapproximately 89% in X Direction. While considering for Y direction the value of 10SSPUM is greater by 6% and for 10SSPLM the valueisgreaterby70%.
For6SSPmodelthevalueof6SSPUMisgreaterby3%and for6SSPLMthemaximumvaluegoesuptoapproximately 36% in X Direction. While considering for Y direction the valueof6SSPUMisgreaterby1%andfor6SSPLMthevalue isgreaterby18%.
4.4
Table4representthevaluesfortimeperiodinbothXandY DirectionforResponseSpectrumAnalysis.
B Models with Upper Bound Stiffness Modifiers
Fromtheaboveprovidedtableitcanbeobservedthattime period reduces in the structures with the below storey podium structure as compared to the basic conventional structure without the inclusion of the podium structure Thus,structuralmodelswithbelowstoreypodiumstructure tendtobestifferthanthemodelwithoutpodium.
Withtheincreaseinthepodiumheight,itisobservedthat timeperiodoftherespectivestructuredecreaseindicating theincreaseinstiffnessoftheoverallstructure.Structural modelswith10storeypodiumshowcasedtheleastvaluefor timeperiodinbothdirectionwhencomparedtoothermodels withreducedpodiumheight.
Models with reduce surface area of podium structure are seen to be having approximately similar values for time periodasthatofmodelswithincreasedpodiumsurfacearea andhavingsimilarheightofpodiumsrespectively.
Models with upper bound stiffness modifiers have approximatelysimilartimeperiodasthatofthemodelswith factored stiffness modifiers but has lesser time period as comparedtomodelswithlowerboundstiffnessmodifiers.
ModelswithLowerboundstiffnessmodifierstendtohavethe highesttimeperiodascomparedtoanyotherstructuredue toreductioninstiffnessofthestructure.
90%massparticipationwasobservedwithintheprovided modesforeverymodelconsidered.
4.5 Backstay Effect
Chart -19, Chart -20, Chart -21, Chart -22, Chart -23 and Chart -24 represent the values in percentage of shear reversedatthepodiumstructureinterfacelevelduetoSPEC XandSPECYintheshearwallwithspandrelidW3.
Accordingtotheobservedtrenditisseenthatthevalueof shear force for the structures with below storey podium reducesdrasticallyatthepodiumstructureinteractionlevel due to increased stiffness of the structure owing to the inclusionofthebelowstoreypodiumstructureascompared tomodelwithoutthebelowstoreyPodiumstructure.
The values of shear force due to SPECX at the podium structure interface level, for the models 10SP, 6SP, 10SSP 6SSPhavereducedto46%,40%,46%,40%ofthevalueof shearforceofthestoreyrightabovethepodiumstructure interface. For SPECY the values have reduced to approximately45%,27%,27%,19%ofthevalueofshear force of the storey right above the podium structure interfacelevelforthesamerespectivemodels.WhileforCS thevaluesarereducedby5%at10th storeyand10%at6th storeyforSPECXand4%and8%forSPECYattherespective floors.
Similartrendisbeenobservedinmodelswithupperbound stiffnessmodifiers.ThevaluesofshearforceduetoSPECXat thepodiumstructureinterfacelevel,forthemodels10SPUM, 6SPUM,10SSPUM,6SSPUMhavereducedtoapproximately 39%,33%,40%and33%ofthevalueofshearforceofthe storeyrightabovethepodiumstructureinterface.ForSPECY the values at the interface level have reduced to approximately41%,20%,30%,10%ofthevalueofshear forceofthestoreyrightabovethepodiumstructureinterface forthesamerespectivemodels.
Formodelswithlowerboundstiffnessmodifiers,thevalues ofshearforceduetoSPECXattheinterfacelevel,formodels 10SPLM, 6SPLM, 10SSPLM and 6SSPLM have reduced to approximately 13%,10%,13%,10%ofthevalueofshear force of the storey right above the podium structure interface. For SPECY the values atthe interfacelevel have reducedtoapproximately19%,19%,15%,9%ofthevalue ofshearforceofthestoreyrightabovethepodiumstructure interfaceforthesamerespectivemodels.
The observed backstay effect is only seen at the podium structureinteractionlevel.
5. CONCLUSIONS
Thebackstayeffectduetopodiumstructureinteractionis beenstudiedinthisreport.Theeffectofinclusionofbelow storeypodiumstructuretothemaintowerisbeenstudied and the results are expressed in terms of lateral displacement, storey shear, storey drift, time period and shear force reversed in shear wall W3. Varying structural configurationwereconsideredofthepodiumstructure by changingtheirheightandsurfacearea.Sensitivityanalysis wascarriedoutusingupperboundandlowerboundstiffness modifiers. From the above provided results following conclusionscanbedrawn.
Backstay effect was observed mainly at the podium structureinteractionlevelandtendtofadeawayaswe moveawayalongtheheightofthestructure.shearforce tends to change its direction at the podium structure interactionlevel.
Thebelowstoreypodiumimposesrestrainonthelower portionoftower,thusrestrictingitsmovement.
It was observed that inclusion of the below storey podiumstructuretendstoincreasethegeneralstiffness ofthestructuresasthevaluesofLateraldisplacement, storeydriftandtimeperiodtendstodecreaseformodels withbelowstoreypodiumstructure
Models with increased number of podium levels, showcaseddecreaseinvaluesforlateraldisplacement, storeydriftandtimeperiod
When models with reduced surface area of podium structurewereobserved,itwasseenthatthevaluesof lateraldisplacementandstoreydriftwerereducingwhen comparedtomodelswithbiggersurfaceareaandsame respectiveheightofbelowstoreypodium.
For sensitivity analysis, models with lower bound stiffness modifiers showcase the maximum values for lateraldisplacement,storeydriftandtimeperiod.
Models with upper bound stiffness modifiers were observedtobealotstifferthanmodelswithlowerbound stiffnessmodifiers.
Baseshearvaluesformodelswithbelowstoreypodium structurewasseentobehigherwhencomparedtobase modelwithoutpodiumbecauseoftheincreasedseismic weight of the structures owing to the inclusion of the podiumstructure.
Models with increased height of podium showcased highervaluesofbaseshearthanmodelswithdecrease heightofpodiumattachedtothem.
Values of base shear tend to decrease for models with smaller surface area of the podium structure as comparedtomodelswithbiggersurfaceareaandsame respectiveheightofthebelowstoreypodiumstructure.
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