ijset.0720140269.1011.0408_dharmesh_1065-1076

SEISMICRESPONSEOFAMIDRISERCCBUILDINGWITHSOFTSTOREYATGROUNDFLOOR

1DHARMESHVIJAYWARGIYA,2Dr.ABHAYSHARMA,3Dr.VIVEKGARG1PostGraduateStudent,CivilEngineeringDepartment,MANIT,Bhopal,Madhya

Pradesh,India,Email:[email protected],CivilEngineeringDepartment,MANIT,Bhopal,MadhyaPradesh,

India,Email:[email protected],CivilEngineeringDepartment,MANIT,Bhopal,MadhyaPradesh,

India,Email:[email protected]

ABSTRACTMany urban multi storey buildings in India today have open ground story as anunavoidable aspect, basically to generate parking or reception lobbies. The upperstoreyshavebrickinfilledwallpanelswithvariousopeningpercentageinthem.Thesetypesofbuildingsarenotdesirableinseismicallyactiveareasbecausevariousverticalirregularities are induced in such buildings which have performed consistently poorduringpast earthquakes. It has been known since long time thatmasonry infillwallsaffect thestrengthandstiffnessof infilled framedstructures. Infillwallsaregenerallyseenasanonstructuralelementandtheireffectisneglectedbyignoringthestiffnessofthe infillwall during themodelling phase of the structure (analysed as a linear bareframe) leading to substantial inaccuracy in obtaining the actual seismic response offramed structures. The objective of the paper is to check the applicability of themultiplication factor of 2.5 for the given building of mid height and to study theinfluenceofinfillstrengthandstiffnessintheseismicanalysisofamidriseopengroundstoreybuilding.Areinforcedconcreteframedbuilding(G+5)withopengroundstoreylocatedinSeismicZoneIVisconsideredforthisstudy.Thisbuildingisanalyzedfortwodifferentcases: (a)consideringboth infillmassand infill stiffnessand(b)consideringinfill mass but without considering infill stiffness by response spectrum analysismethod. The result shows that the effect of infills stiffness on structural response issignificantunderlateralloads.Themagnificationfactorof2.5ishightobemultipliedto

DHARMESH VIJAYWARGIYA et al.

DATE OF PUBLICATION: AUGUST 09, 2014

ISSN: 2348-4098

VOLUME 02 ISSUE 06 JULY 2014

INTERNATIONAL JOURNAL OF SCIENCE, ENGINEERING AND TECHNOLOGY- www.ijset.in 1065

columnforcesofthegroundstoreyofthegivenmidriseopengroundstoreybuilding.Itisfoundthattheinfillpanelsincreasesthestiffnessoftheupperstoreysofthestructure,thereby increasing the forces, displacement, drift and ductility demand in the softgroundstorey.Thiscouldbecomethecauseoffailureofopengroundstoreybuildingsduringearthquake.

INDEXTERMS:Opengroundstorey,masonryinfillwalls,nonstructuralelement,bareframe,infillstiffness.

1. INTRODUCTIONReinforced concrete framed buildingshave become common form ofconstruction in urban and semi urbanareas around the world which ismasonryinfill.Numeroussuchbuildingsconstructed in recent times have aspecialaspectthegroundstoreyisleftopen, whichmeans the columns in thegroundstoreydonothaveanypartitionwalls between them. These types ofbuildingshavingnoinfillmasonrywallsingroundstorey,buthaving infillwallsin all the upper storeys, are called asOpen Ground Storey (OGS) Buildings.Thisopengroundstoreybuildingisalsotermed as buildingwith Soft Storey atGroundFloor.

There is significant advantage of suchtype of building functionally but whenseismicperformancepointofviewsuchbuildingisconsidereditisfoundtohaveincreased vulnerability. The openground storey buildings are generally

designed as framed structures withoutregard to structural contribution ofmasonry infill walls. The presence ofinfillwallsinalltheupperstoriesexceptin the ground storey makes the upperstoriesmuchstifferascompared to theopen ground storey. Thus the upperstoriesmovealmosttogetherasasingleblock and most of the horizontaldisplacement of the building occurs inthe soft ground storey itself and hencethe ground storey columns are heavilystressed.IS1893(2002)recommendsamagnificationfactorof2.5tobeappliedon bending moments and shear forcesin the columns of ground storeycalculated for the bare frame underseismicloads.

The salient objectives of the presentstudy have been to study the effect ofinfill strength and stiffness in theseismic analysis of open ground storey(OGS) buildings, to check theapplicabilityof themultiplicationfactor



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of2.5asgivenintheIndianStandardIS1893:2002fordesignofamidriseopengroundstoreybuildingandtoassesstheinfluence of varying the infillarrangementsontheanalysisresultsbytaking various combinations of infillthickness, strength, modulus ofelasticityandopenings.

2. DESCRIPTION OFSTRUCTURALMODEL

2.1GEOMETRY

For thestudy fivedifferentmodelsofasix storey building are considered. ThebuildinghasfivebaysinXdirectionandfour bays in Y direction with the plandimension22.5m14.4mandastoreyheightof3.5meachinallthefloorsanddepthoffoundationtakenas1.5m.Thebaywidthalonglongitudinaldirectionis4.5m and along transverse direction is3.6m.Thebuildingiskeptsymmetricinboth orthogonal directions in plan toavoid torsional response under lateralforce. The column is kept square andsize of the column is kept samethroughouttheheightofthestructuretokeepthediscussionfocusedonlyonthesoftfirststoreyeffectwithoutdistractedbytheissueslikeorientationofcolumn.Thebuildingisconsideredtobelocated

in seismic zone IV and intended forresidentialuse.

2.2MATERIALPROPERTIES

M25 grade of concrete and Fe415gradeofreinforcingsteelareusedforalltheframemodelsusedinthisstudy.Theunit weights of concrete and masonryare taken as 25.0 kN/m3 and 20.0kN/m3 respectively. The modulus ofelasticity of the bricks found in Indiavaries from 350 MPa to 5000 MPa. Torepresent the extreme cases of strongandweak infillwalls2 combinationsofinfillwallsareconsideredformodelling.Thethickerwallof230mmthickness iscombinedwith strong infillwall havingE = 5000 MPa and thinner wall of115mm thickness is combined withweakinfillwallhavingE=350MPa.Thepoison ratio of concrete is 0.2 and ofmasonryis0.15.

Figure1:Planofthestructure



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3. MODEL CONSIDERED FORANALYSIS

Following five models are analyzedusingresponsespectrumanalysis

i) Model I: Bare frame model(reinforced concrete frame takinginfill masonry weight, neglectingeffectofstiffness).

ii) Model II: Building with strong infill(effectofstiffnessisalsoconsideredinadditiontotakingweightofinfill).

iii) Model III:Buildingwithstrong infillhaving openings (model II withopeningsatcertainpanels).

iv) Model IV: Building with weak infill(effectofstiffnessisalsoconsideredinadditiontotakingweightofinfill).

v) Model V: Building with weak infillhaving openings (model IV withopeningsatcertainpanels).

Figure2:ModelI:Bareframe

(a)Frontelevation

(b)Sideelevation

Figure3:ModelII&IVInfilledframes

(a)Frontelevation



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(b)Sideelevation

Figure4:ModelIII&VInfilledframeswithopenings

4. MODELLING OF FRAMEMEMBERS AND INFILLWALLS

The structural members are modelledwith the aid of commercial softwareETABS v 9.7.1 in compliance with thecodes IS 4562000 and IS 18932002.The framemembers aremodelledwithrigid end conditions. The floor slabswere assumed to act as diaphragms,which ensure integral action of all thelateral loadresisting elements. Thefloor finish on the floors is taken to be1.0 kN/m2. The live load on floor istakenas3.0kN/m2andthatontheroofto be 1.5 kN/m2. In seismic weight

calculations,25%ofthefloorliveloadsareconsideredintheanalysis.

For an infill wall located in a lateralloadresisting frame, the stiffness andstrengthcontributionoftheinfillhastobe considered. Nonintegral infill wallssubjected to lateral load behave likediagonal struts. Thus an infill wall canbe modelled as an equivalentcompressiononly strut in thebuildingmodel. Rigid joints connect the beamsandcolumns,butpin jointsconnect theequivalentstrutstothebeamtocolumnjunctions. The length of the strut isgivenbythediagonaldistance(d)ofthepanel and its thickness is equal to thethickness of the infill wall. The elasticmodulus of the strut is equated to theelasticmodulusofmasonry(Em).Smith(1966)proposed a formula to calculatethewidthofstrutbasedontherelativestiffnessoftheframeandtheinfillwalls.

5. RESULTSANDDISCUSSION5.1 BENDING MOMENT AND SHEARFORCEINCOLUMNS

As can be seen from the tables 1 & 2(model II to V) and figures 5 to 8 thebending moments and shear forces(strength)demandsareseverelyhigherfor the ground storey columns withrespect to first storey columns, in case



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ofthesoftgroundstoreybuildingswhenthey are analyzed by considering infillas structural component taking intoconsideration their stiffness also withtheir weight. The introduction of wallsin the first storey (model II to V)reduces the force in the first storeycolumns. In model I, the bendingmoment and shear forces are themaximumascomparedtoothermodels,as there is no effect of infill wallsconsidered in their analysis whichshowstheforcedemandsdependsuponthe stiffness of the members. Also theforces in the first storey columns ofmodelIarealmostequaltotheforcesinthe ground storey columns or evenmore for shear forces which isdrastically opposite behaviour ascompared to the other models.Therefore the importance of modellingand considering the infill walls asstructural component and also thedescriptionofinfillmaterials,theirtype,strength and their elastic modulusdefinitionisrealizedhere.

Table1:Maximumbendingmomentingroundstoreyandfirststoreycolumns

MaximumBendingMoment(kNm)

Model

Longitudinal

Transverse

GroundStorey

FirstStorey

GroundStorey

FirstStorey

I 79 74 77 73

II 86 48 90 40

III 82 51 84 36

IV 70 27 71 28

V 66 26 68 28

Figure5:Comparisonofmaximumbending

momentsinlongitudinaldirection



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Fig 6: Comparison of maximumbending moments in transversedirection

Table 2: Maximum shear force ingroundstoreyandfirststoreycolumns

ShearForce(kN)

Model

Longitudinal

Transverse

GroundStorey

FirstStorey

GroundStorey

FirstStorey

I 40 42 40 41

II 51 21 52 15

III 49 21 51 14

IV 39 15 40 18

V 36 15 38 18

Fig 7: Comparison of maximum shearforceinlongitudinaldirection

Fig 8: Comparison of maximum shearforceintransversedirection

5.2LATERALDEFORMATIONTable3:Displacement(inmm)inlongitudinaldirection

Model Storey1 Storey2 Storey3 Storey4 Storey5 Roof



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I 7.8 15.9 23.7 30.4 35.3 38.0

II 8.1 9.3 10.3 11.1 11.8 12.2

III 7.9 9.1 10.2 11.0 11.7 12.2

IV 6.8 10.7 13.8 16.6 18.7 19.9

V 6.6 10.4 13.6 16.4 18.5 19.8

Figure9:Displacementprofilealonglongitudinaldirection

Table4:Storeydrift(inmm)inlongitudinaldirection

Model Storey1 Storey2 Storey3 Storey4 Storey5 Storey6 Roof

I 0.6 2.0 2.3 2.2 1.9 1.4 0.8

II 0.8 2.0 0.36 0.28 0.24 0.19 0.12

III 0.8 1.9 0.37 0.30 0.26 0.20 0.14

IV 0.60 1.7 1.0 0.90 0.78 0.60 0.35

V 0.60 1.6 1.1 0.92 0.80 0.62 0.36



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Figure10:Storeydrifts

The displacement of the model I at allthe floors is themaximumwithrespecttothatofalltheothermodels.Thereisahuge difference between thedisplacement values of model I and allother models. This gap of difference isincreasing uniformly with the increasein the storey level. Also thedisplacement of model IV & V is morethan thedisplacementofmodel II& IIIthroughoutthefloors.Thedisplacementof model I is of such amount becausethere is no lateral stiffness provided tothestructurebytheinfillwall.

As can be seen from figures and tablesforstoreydrift,thestoreydriftprofileofmodel I is smooth throughoutwhereasformodelIItoVthestoreydriftchanges

abruptly from ground storey to firststorey. This sudden change of slope ofstoreydriftprofilealongprofileofeachmodel signifies stiffness irregularitybetween soft storey and infilled storey,encountered because of modellingstiffness of infill wall for soft groundstorey buildings. Such stiffnessirregularity of soft ground storeybuildingsiscriticalfromfailurepointofview when subjected to earthquakeforces because of resemblance of itsbehaviour with the behaviour ofinverted pendulum. The upper storeysmove together as a single block andmost of the horizontal deformation ofthe building occurs in the soft groundstoreyitself.

5.3MAGNIFICATIONFACTOR

Table5indicatesthatthemagnificationfactor values is found to vary between0.88 to 1.17 for the bending momentand for shear forces between 0.95 to1.33inthegroundstoreycolumnsofthemodels II to V in comparison to thecorresponding values of bendingmoment and shear force in the groundstoreycolumnsofmodelI(bareframe).



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Table5:Magnificationfactorsforbendingmomentandshearforce

Model I II III IV V

Maximum BM ingroundstorey(kNm)

ExteriorColumn

76 89

(1.17)#

83

(1.09)

71

(0.93)

68

(0.89)

InteriorColumn

77 90

(1.17)

84

(1.09)

71

(0.92)

68

(0.88)

Maximum shear forceingroundstorey(kN)

ExteriorColumn

39 52

(1.33)

51

(1.31)

40

(1.03)

38

(0.97)

InteriorColumn

40 52

(1.30)

50

(1.25)

40

(1.0)

38

(0.95)

# Magnification factor values for bending moment & shear force obtained by dividing with thecorrespondingvaluesforthebareframe.

6. CONCLUSIONSThe following are the main findings ofthepresentstudy

i) The structural member forces,deformationsdovarywiththedifferentparameters associated with the infillwalls. Such variations are notconsidered in current codes and thustheguidanceforthedesignofbuildingshaving infill walls is incomplete andspecifically for buildings with softground storey it is imperative to havedesignguidelinesindetail.ii) Infill panels increases thestiffness of the structure and theincrease in the opening percentage

leads to a decrease on the lateralstiffness of infilled frame. Hencebehaviour of building varies with thechange in infill arrangements. Thisindicates that modelling of reinforcedconcrete frame building without infillwall (panel) or bare frame model maynotbeappropriatefortheanalysis.iii) The analyses result shows thatcolumn forces at the ground storeyincreases for the presence of infillwallin the upper storeys. But design forcemagnification factor found to be muchlesser than2.5.This isparticularly truefor midrise open ground storeybuildings. It is seen from responsespectrum analysis that the



ISSN: 2348-4098



magnificationfactordecreaseswhenthestiffness of infill panels are decreasedeither by reducing infill strength(thicknessandmodulusofelasticity)orby providing openings in the infillpanels.iv) When a bare frame model issubjected to lateral load, mass of eachfloor acts independently resulting eachfloor to drift with respect to adjacentfloors.Thusthebuildingframebehavesin the flexible manner causingdistribution of horizontal shear acrossfloors. Inpresenceof infillwall (panel),the relative drift between adjacentfloors is restricted causingmass of theupper floors to act together as a singlemass. In such case, the total inertia ofthe all upper floors causes a significantincrease in horizontal shear force atbase or in the ground floor columns.Similarlyincreasesthebendingmomentinthegroundfloorcolumns.v) From the present results it isfound that, lateral displacement is verylarge in case of bare frame as comparetothatof infilledframes. If theeffectofinfill wall is considered then thedeflection has reduced drastically. Thepresence of walls in upper storeysmakes them much stiffer than openground storey. Hence the upper storeymove almost together as a single block

andmostofthehorizontaldisplacementofthebuildingoccursinthesoftgroundstoreyitself.

REFERENCES[1]. Agarwal P. and Shrikhande M.(2006). Earthquake resistant design ofstructures. PHILearningPvt.Ltd.,NewDelhi.

[2]. Arlekar J.N., Jain S. K. and MurtyC.V.R (1997). Seismic response of RCframesbuildingswith soft first storeys.Proceedings of CBRI golden jubileeconferenceonnaturalhazards inurbanhabitat,NewDelhi.

[3].DavisR.,MenonD.andPrasadA.M.(2008). Evaluation of magnificationfactorsforopengroundstoreybuildingsusing nonlinear analyses. The 14thWorld Conference on EarthquakeEngineering,Beijing,China.

[4]. ETABS nonlinear version 9.7.1.ExtendedThreeDimensionalAnalysisofBuilding Systems, Users Manual.Computers and Structures, Inc.,Berkeley,California,USA.

[5]. IS 1893 Part 1 (2002). Criteria forEarthquake Resistant Design ofStructures.Bureauof Indian Standards,NewDelhi.



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[6]. IS456(2000).Plainandreinforcedconcrete: Code of practice. Bureau ofIndianStandards,NewDelhi.

[7]. SubramanianN. (2004). DiscussiononseismicperformanceofconventionalmultistoreybuildingwithopengroundfloorsforvehicularparkingbyKanitkarand Kanitkar. The Indian ConcreteJournal.78,1113.

BIOGRAPHIES

DharmeshVijaywargiya,PostGraduate Student, CivilEngineering DepartmentMANIT, Bhopal, MadhyaPradesh,India

Dr. Abhay Sharma, AssociateProfessor, Civil EngineeringDepartment MANIT, Bhopal,MadhyaPradesh,India

Dr. Vivek Garg, AssistantProfessor, Civil EngineeringDepartment MANIT, Bhopal,MadhyaPradesh,India



ISSN: 2348-4098



ijset.0720140269.1011.0408_dharmesh_1065-1076

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