EFFECT OF SOIL STRUCTURE INTERACTION ON DYNAMIC BEHAVIOUR OF STRUCTURES- A REVIEW

International Research Journal of Engineering and Technology (IRJET) e ISSN: 2395 0056

Volume: 09 Issue: 07 | July 2022 www.irjet.net p ISSN: 2395 0072

EFFECT OF SOIL STRUCTURE INTERACTION ON DYNAMIC BEHAVIOUR OF STRUCTURES- A REVIEW

1M. Tech Scholar Techno India University, Kolkata (WB)

2HOD, Dept. of Civil Engineering, Techno India University, Kolkata, West Bengal, India ***

Abstract - Response of soil which results motion within the structure during seismic activity is usually overlookedas most of the structures are assumed to be resting on fixed base, which sometimes leads to unsafe design after doing post failure analysis. Observations from some of the past seismic events such as 1989 Loma Prieta earthquake and 1995 Kobe earthquake where SSI is one of the main reasons for the collapse of the Hanshin Expressway show evidences of adverse nature of SSI in certain circumstances. Soil flexibility shouldbe considered especially during the analysis of high rise buildings or structures resting on soft soil or located in high seismic zones to avoid any sort of failure and ensure safe service. This study is at the growing stage, given its complexity and inadequate detailed guidelines to calculate effect of SSI within the standard codes which should be advanced with easier methods to resolve soil structure interaction problems with greater ease in the coming future. Current paper attempts to review the state of art about various methods of soil structure interaction analysis conducted by various researchers using some of the popular finite element analysis softwares and some of the provisions mentioned in different International seismic codes.

Key Words: Soil Structure Interaction (SSI), fixed base, flexible base, seismic loading, soil flexibility, Kinematic interaction,Inertialinteraction

1. INTRODUCTION

All structures are built over soil and at times maybe subjected to seismic force during an earthquake, the intensitydependingonseismiczones.Thewavesthatarrive atfoundationduringanearthquakeproducemotionsinthe structureitself.Motionsdependonthestructuralorbuilding layout&thevibrationalcharacteristics.Forthestructureto responsetothemotion,itneedstoovercomeitsowninertia, whichresultinaninteractionbetweenthestructureandthe soil. Such an interdependent behaviour between soil and structureregulatingtheoverallresponseisreferredasSoil StructureInteraction(SSI)behaviourinthepresentcontext. Soil structure interaction broadly can be divided into two phenomena:

a)kinematicinteraction

b)inertialinteraction

Earthquakegroundmotioncausessoildisplacementknown asfree fieldmotion.However,thefoundationembeddedinto thesoilwillnotfollowthefreefieldmotion.Thisinabilityof the foundation to match the free field motion causes the kinematicinteraction. On the other hand, the mass of the superstructure transmits the inertial force to the soil, causingfurtherdeformationinthesoilistermedasinertial interaction.

It is often seen that to carry out soil structure interaction analysisinFEMsoftwareisrepresentedasmatfoundation consideredasaslabrestingonequivalentlyspacedsprings ofsuitablestiffnessvalue.Conventionalpracticeconsiders the analysis of structure and foundation separately, assumingthatthebaseofstructureisfixedmeansthebase of the foundation transfers the load by direct bearing on solid rocky stratum or soil and subsequently load distributingwithinthebuildingframesarecalculated.This assumptionisapplicablewhenthesuperstructureishighly flexible than the underlying soil stratum upon which the foundationrests.SSIplaysacriticalrolewhenthebehaviour ofstructureisconsideredunderstaticordynamicloading.It influencesthebehaviourofsoil,aswellastheresponseof pile under loading. The analysis is highly essential for predicting a more accurate structural behaviour so as to improve the safety of structures under extreme loading conditions.

2. HISTORY OF SSI

Soil Structure Interaction is an interdisciplinary field of study which lies at the intersection of soil and structural mechanics, soil and structural dynamics, earthquake engineering, geophysics and geomechanics, computational andnumericalmethods,andvariousothertechnicalfieldof study.Itsorigincanbetracedbacktothelate19thcentury which evolved and further advanced gradually in the upcoming decades and also in the first half of the 20th centuryandprogressedrapidlyinthesecondhalfofthe20th centuryacceleratedmainlybytheneedsofthenuclearpower and offshore industries and with the launch of powerful computersandsimulationtoolssuchasfiniteelementswith the need for improvements in seismic resistant design of structures.

Roesset[37]andKausel[38]presentedreviewsoftheearly stage developments in field of Soil Structure Interaction after

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extensiveresearch.InadditiontothetwocomponentsofSSI kinematicandinertialoriginallycoinedbyWhitman,Roesset [37] also discussed about the direct and substructure approaches to perform SSI analysis. He also reported previousworksbyReissnerandBycroft,VeletsosandWei, LucoandWestman,andNovakinfieldofdynamicstiffnessof foundations,aswellaseffectsofdepositsintheformoflayers or strata, embedment and pile group. On the other hand, Kausel [38] presented the development in SSI sequentially withstartingfromfundamentalsolutionscommonlytermed as green’s functions proposed by mathematicians and scientists’ way back in early 19th century. He reported notable contributions in static SSI by Boussinesq, Steinbrenner,ReissnerandHansonjusttonameafew.Erich Reissner,in1936,putfoundingstonesfordynamicSSIwitha publicationwhereheexploredthebehaviourofcirculardisks on elastic half spaces subjected to time harmonic vertical loads, which was further carved by notable work done by Luco, Bycroft, Housner, Newmark, Veletsos, Whitman and many others. Kausel[38] himself initiated development of substructure approach to solve SSI problems. But the beginning of the modern era and rapid growth of Soil StructureInteractiontookplacesomefourdecadesagowith the publication of some renowned influential papers by VeletsosandWeiin1971andLucoandWestmannin1971 and1972respectively,whichprovidedacompleterigorous solutiontotheproblemofcircularplatesunderlainbyelastic half spaces excited dynamically over a wide range of frequencies,andforawidesetofPoisson’sratios.

3. IMPORTANCE OF SSI

ItisusuallyseenthatconsideringSSIprovestobebeneficial onseismicresponseofastructuresincestructuresanalysed usingSSIaremoreflexibleincreasingitsnaturalperiodand dampingratiothusleadingtoareductioninbaseshearofa structureascomparedtoastructurewithfixedbase.Dueto these reasons SSI has always been ignored by structural engineerstoreducethecomplexitiesinvolvedintheanalysis. But, observations from effect of 1971 San Fernando earthquake and 1989 Loma Preita earthquake shows a different story. Badry et al[39] conducted SSI analysis for asymmetrical buildingssupported on piled raft whichwas damagedduring2015NepalEarthquakewhichwasobserved thatadverseeffectsofSSIcanincreasewithasymmetry in geometryofsuperstructure.

RayChowdhury[40] highlightedthepossibilityofdifferential settlement arising out of soil flexibility for low rise steel moment resisting building frames where SSI needs to be carefullyappliedforheavilyloadedfootings owing tohigh inertial effects. Hence there is a compulsion to develop a rational basis for seismic design incorporating SSI. Saez et al[40] researchedontheinfluenceposedbydynamic SSIon inelastic response of moment resisting frame buildings founded on dry and fully saturated sands. They showed a noticeable effect of dynamic SSI in case of fully saturated

sands owing to the increase in pore water pressure. This suggeststhattheimportanceoftakingSSIintoaccountcan vary depending on site conditions. Contrary to elastic structures, SSI can increase ductility demands and total displacements in case of inelastic structures. Given that structuresareexpectedtobeinelasticintheeventofsevere earthquakes,thecurrentseismicprovisionsareinadequate. Jarernprasert S. et al[42] has also designed an approach to integrateSSIbyusingamodifiedseismicdesigncoefficient thatallowsthestructuretoreachitstargetductility.Itcan thereforebeobservedthatSSIneedstobetakenintoaccount whendesigningnon elasticstructures.Despiteconsiderable amount of research in this field of SSI but considering SSI whiledesigningreallifestructuresatsiteveryraregiventhe inadequateprovisionofSSIinstandardseismiccodes.Thus, it is necessary to develop a procedure to analyze the SSI probleminasimplewaybutquiteprecise.

4. CURRENT PRACTICE OF SSI RESEARCH

Several researchers worked on the response of soil structure interaction in framed structures and also the influence of structure under dynamic loading considering parameterssuchasstorydrift,baseshear,etc.

Elasto plastic interaction analysis of two bay, two storey planeframewithfoundationbeam soilsystemdevelopedby Hora M [1] using the finite element method to make superstructurebehaveinlinearelasticmannerandthesoil mass in elasto plastic manner according to various yield criteria. Settlements in soil mass, contact pressure below foundation beam, forces in the frame members and the foundation beam are evaluated and collapse load is determinedsinceanalysinginteractionsysteminthisway, yieldsarationalstructuralbehaviourastheshearforcesand bending moments get significantly altered due to the resulting differential settlements of soil mass. Meanwhile, forces and moments get transferred from the exterior columns towards the interior ones due to elasto plastic interaction analysis and soil remaining in elastic state, although the soil mass below the outer edges has fully yielded.

Two numerical models where model 1 simulates seismic soil structureinteractionincludingthestructure,foundation modeled with finite elements and the subsoil conditions modeled with springs and dampers whereas model 2 is generatedforcomparisonpurposesconsideringfixedbase weredevelopedbyusingANSYSsoftwareofa6storiedRC building with basement to study the influence of soil structureinteractionbyGarciaJ.A.[2].Resultsshowthatupon consideringtheinfluenceofsoil structureinteractioninthe dynamicbehaviourofthestructurereflectsinanincreasein the vibration period as well as an increase in system damping compared to the fixed base model. Moreover, economicdesignsareobtainedbyincludingthesoilinthe structural analysis and design as there is a reduction in seismicloads.

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Analyzingabuildingbyvaryingnumberofstoriesas5,10,15 by Kumar D.N. et al[3] and Kumar A. et al [4] of a 30 storey building with both defining the soil medium by springs consideringeffectoffoundationsoilsettlement withmain objectivetostudytheeffectofsoil structureinteractionon horizontal displacement of each floor and vertical displacementatthesupportsandthebendingmomentand shearforcesattheinteriormiddleframeofbuilding.Wind loadingistakenas55m/s,50m/s,33m/sby [3]withboth varyingsubgrademodulusoffoundationsoilfrom12,000 kN/m3 to 60,000 kN/ m3. It was put forward that with decreasingsubgrademodulus,displacementincreasesinx& y directions and horizontal displacement & vertical displacementincreaseswithincreaseinnumberofstories& so the effect of soil structure interaction has to be consideredespeciallyforlowersubgrademodulusofsoilat higherseismicintensities.

Usually, flexibility of the soil results in the decrease of stiffness which in turn increases the natural period of the structure, so Bhojegowda et al[5] using ETABS software studied the effect of soil flexibility on the natural period besidesidentifyingspringstiffnessfordifferentregularand irregular storey buildings with isolated, mat and pile foundationsfordifferentsoilconditions.Responsespectrum analysis has been adopted which showed an increase of bendingmomentanddisplacementfromfixedbaseanalysis toflexiblebaseanalysis,butnotmuchvariationfor15storey framewithpilerestingonhardandmediumsoilcompared to 5,10 storey frame as framed structure with pile foundationbehavesasafixedsupportforhomogeneousnon liquefiablesoil.Baseshearformatfoundationincreasesas famed structure with mat foundation possesses high foundationstiffnessincomparisonwithisolatedfoundation but when other parameters are compared with isolated footing displacement, bending moment and time period reduces.

Analysing the dynamic behaviour of building frames over raftfootingwithandwithoutsoilflexibilitywherethesoil beneathraftfootingisatruesoilmodelorcontinuummodel and to evaluate the responses in terms of lateral natural period,seismicbaseshear,lateraldisplacementorstorydrift wasthemainfocusofKuladeepuMNetal[6]. Spaceframe, foundation and soil was taken part of a single compatible unitwithsoilmodelconsideredashomogeneous,isotropic andelasticofhalfspacetakingdynamicshearmodulusand poisson’s ratio as the inputs in SAP2000 software. The observations indicate that considering the soil structure interactionaswellasincreaseinthenumberofstories,the fundamental natural period and base shear increases in a structureresultinginthedecreaseofshearmodulusofsoil. Butwith the reductionof shearmodulus ofsoilas well as number of stories, the maximum lateral displacement is foundtobeincreasing.

Modelling soil by Winkler approach or spring model and elasticcontinuumapproachtermedasFEMonanirregular 15 storey with analysis carried forward using SAP2000 buildingbyNiravetal[7]tostudytheeffectofsoilflexibility. The analysis concluded replacing fixed base by spring or modellingsoilasaFEM,changeintheresponseofstructure isobservedincaseofsoftsoilandthebaseshearincreases comparedtothespringandFEMmodel,butdisplacement increasesinXandYdirectionofaFEMmodelcomparedto spring and fixed base models with an increase in soil flexibility.

To incorporate the effect of soil flexibility of significant designvariables,G.V.RamaRaoetal[8]studiedontheseismic responseofchimneystructureswithraftfootingusingstaad pro v8i software. Since there is a large difference in the foundationinputmotionduringtheearthquakeforsoftsoils compared to free field ground motion which exists in the absenceofstructurewiththeassumptionofafixedsupport resultinginsignificantvariationinfrequencyandamplitude valuesofstructuresduringactualseismicactivityfromwhat the analysis provides treating the structure to be on fixed base.Thefinalanalysisshowsdecreasingtrendoflateral& support displacement with increase in soil subgrade modulus also increase in soil flexibility decreases natural frequencysignificantly.

Toassessthedifferencesinthedesignresponseandanalysis outputs arising due to inconsideration of soil structure interactionintheanalysisforreinforcedconcretebuilding framessupportedonpilefoundationandembeddedinloose sand using the Open SEES program where five types of analysishasbeencarriedouttoestimatethedifferentdesign response and analysis output parameters being the main focusofstudyofSharmaNishantetal[9] Hehighlightedthe factthatitmaynotalwaysbefeasibletoignoretimehistory analysisincaseswheresiteresponseinfluencestheoverall responseofthebuilding foundation soilsystem.Also,lowest estimateofforces&driftvaluesisfoundbyequivalentstatic methodbutfixedbaseanalysisusingspectrumcompatible groundmotionshowsthelargestvalues.Estimatesofforce obtained using linear soil structure interaction and non linearsoilstructureinteractionaresignificantlylesserthan thatobtainedbyfixedbaseanalysisbutslightlylargerthan equivalentstaticmethod. Although effectofsoildeposit is considered here by modifying the ground motion, direct applicationofmotionisnotrecommendedforestimatingthe designforces.Instead,thefoundationsoilmediumshouldbe modelled to obtain the realistic estimate of design forces. Hence,EquivalentStaticMethod&FixedBaseAnalysismay notalwaysprovidearealisticestimate.

To determine the change in various seismic response quantitiesduetoconsiderationofflexibilityofsoilandthe effectofseismiczones,VenkateshM.B.etal[10] dideffective modelling of a multiple bay regular RC building of eight storeys with the soil beneath the structure modelled as

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equivalentsoilspringsconnectedto theraftfoundationin staadprosoftwarethereby carryingoutmodalanalysisof buildingsystemandresponsespectrumanalysisofthesoil structuremodel.Theresultsshowedanincreaseinnatural periodwithsoilflexibilitybutreductioninbaseshearisseen forflexiblebasecomparedtofixedbaseanalysis.Anincrease inresponseofstructurewithchangeinsoiltypefromhard tosoftwithchangeofseismiczonefromIIItoVirrespective ofheightofstructure.Also,asignificantvariationisobserved in raft shear stress and bending moment for soft and medium soil type compared to hard soil depicting considerablesettlementofraftinsoftsoil.

RC building of 20 storey with three types of soil is considered resting on raft foundation underneath incorporating the effects of soil flexibility based on shear modulus, poisson’s ratio and modulus of elasticity. Patel Bhaviketal[11] studiedsoilstructureinteractionanalysisof thesameusingequivalentstaticmethod,responsespectrum methodandtimehistorymethodforrigidbaseandflexible base. Actual record of accelerogram of bhuj earthquake is invoked for time history method, carrying out the entire analysisinSAP2000software.Resultsconveyedmodelling the soil as solid object gives more deflection compared to whensoilismodelledasspring.Observationisalsomadefor baseshearwhichisalmostsamewhensoilismodelledas spring but a significant difference is seen when soil is modelledassolidbutbaseshearforflexiblebasedecreases ascomparedtofixedbase.

Analyzing the effect of soil structure interaction on the elastic response of moment resisting framed reinforced concretestructuresfoundedonembeddedraftinanelastic half space to accelerograms compatible to design spectra carried out by Anand Vishwajit et al[12] with a need to understand elastic response of structure soil systems subjected to harmonic excitations and Design Basis Earthquake obtained using the substructure approach. AnalysiswasperformedonMATLAB2015usingNewmark γ β method which was adopted for analysis thus showing soil structureinteraction makes an elastic structuremore flexible leading to an increase of natural time period and damping of the structure soil system. There is a reduced base reaction in case of structures which exhibit elastic behaviourunderdesignbasisearthquakesconsideringSSI.

Replacingthefixedsupportatbasebyaspringofequivalent foundation stiffness of a multistoried reinforced concrete residential building frame supported on isolated footings foundedondifferenttypesofsandysoillocatedinseismic zone V to perform flexible support analysis by researcher Verma V.K. et al[13]. Significant maximum vertical and differential settlement between footings is observed in flexiblesupportanalysis.Compressibilityofsoilleadstothe redistribution of the forces in beams and columns and reversal in the nature of these forces thereby increasing verticalsupportreactionandsupportmomentforabuilding supportedonthelessstiffsoil.

To study the effect of strong ground motion on joint displacement, axial force and time period and mass participatingfactorsonaG+10reinforcedconcreteirregular multi storeyed building, Singh S.K. et al[14] utilized three differentsoftwarenamelyETABS,STAADPRO&SAP2000as perdesignresponsespectrumcurvesuggestedbyIS1893 1[31] to perform dynamic analysis. Based on the response spectra study, he highlighted that the modal mass participatingfactorismorethan75%inthehighermode& the considered structure is stiff for earthquake excitation. Thefrequencyinfirstmodeofvibrationisfoundbetween 0.44 Hz to 0.57 Hz by different programs, which shows buildingmuchstiffer.Meanwhile,thejointdisplacementin X directionisfoundmoreascomparedtoYandZdirections duetothefactthattheearthquakemotionwasappliedinX direction which depicts uplift in Y direction and displacementinZ direction.

Theeffectofvariationinslopeanglesof15,30,45degreesfor a15mheightbuildingrestingonslopingground,considering fixed base and taking soil structure interaction into consideration was done by Ghosh Rahul et al[15] implementing equivalent static force method, response spectrum method, time history method, nonlinear static method & nonlinear time history method. While, linear analysisisconductedusingETABSsoftwarebutnonlinear analysis using SAP2000 software. Results of the analysis indicate that structures resting on sloping ground reflects differentialmovementoneithersideofthestructuresince the taller side moves more than the shorter side in the directionofforceindicatingstiffnessconcentrationonthe shortersideofthestructureonthehigherleveloftheslopes. Thus, the columns on the higher side of the slope are subjected to heavy torsional force and also subjected to increased bending moment due to reduction of column height. Meanwhile, bending moment of the columns on shorter side of the structure at higher level of slopes increaseswiththerisingstoreylevelaswellasslopeangle, even if there is no reduction of column length. It was also observed for structures analyzed without considering soil structureinteraction,forcesareoverestimatedsuchasbase shearandbendingmomentbutunderestimatesresponses suchastimeperiod,displacementandtorsion.

Comparative study of a G+10 building resting on sloping ground at an angle of 0,10,20 degrees and plane ground carriedoutbyManjunathHVetal[16] usingETABSsoftware performingequivalentstaticanalysisandresponsespectrum analysistodoacomparisonofresultsofdisplacement,story drift, base shear between IS code1893:2002 and IS code 1893:2016. Analysis results shows an increase in displacement and story drift values for models analyzed usingIScode1893:2016ascomparedtoIScode1893:2002 whereasdisplacementandstoryshearvaluereduceswith increaseinslopingangle.Also,itisobservedthatbaseshear is lesser for models analyzed using IS code 1893:2016 comparedtoIScode1893:2002.

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Municipalsolidwastefinerfraction(MSW FF)utilizedasa sustainable structural fill material to improve the bearing capacity and reduce settlement below foundations supportedonweaksoilsorwhenfoundationsaresupported on filled up soil & also to evaluate the bearing capacity, settlementandmodulusofsubgradereactionfortheshallow foundationofdifferentsizesandshapesrestingonsoilwith low subgrade modulus and with a layer of MSW FF as structuralfill.Tounderstandtheeffectofsoilandfoundation stiffnessonthebasepressure,settlement,bendingmoment and shear stress in shallow foundations for both cases of naturalsoil&withMSW FFasstructuralfill,PatilM.etal[17] undergoneasoil structureinteractionanalysisusingstaad prosoftware.Fromthesoil structureinteractionanalysis,it is observed that the effect of soil subgrade modulus on foundationbasepressureandsettlementisprominent,while having a negligible effect of bending moment and shear stress in the foundation. Further, the relative stiffness of foundationandsoilhasasignificanteffectonthefoundation designparameters,whichshowedapromisingpotentialof utilizingMSW FFasasustainablestructuralfill.

Consideringeffectofinfillstiffnessusingmodellingapproach givenbyHendryforfixedbaseandflexiblebase,aG+7infill masonry RC building is considered. Rajput Harsh et al[18] carried out equivalent static analysis for the two building modelsand tocomparethe performance ofinfill masonry building resting on raft foundation with fixed base and flexiblebasealongwithsoilstructureinteractionforseismic loading considering the parameters of story shear, floor displacement,storydrift,timeperiodandsettlementofraft usingstaadprosoftware.Aftertheanalysisiscarriedoutit wasreportedmaximumvalueofstoryshearforflexiblebase about 1.23 times of fixed base infill frame model. Floor displacementisabout4.03to5.04timesoffixedbaseinfill framemodelwhereas,storydriftisabout1.26to4.86times of fixed base model. Besides, the study conveyed, considering the soil structure interaction in dynamic analysis of RC building frame the time period increases about1.7timesoffixedbaseinfillmodelandsettlementto increaseby62.07mmmorethanfixedbaseinfillmodelwith a differential settlement of magnitude 5.28 mm occurring betweencentreandcornerofraftfoundation.

Two support conditions mainly, fixed support and other supportedonelasticmatsupportedonsoilspringsproposed byWinkler,whereamatfoundationwiththickness850mm furtherrestingonthreetypesofsubgrademodulusofsoilon a 10 storied RC building. Alkari A.K. et al[19] analyzed the same using Response Spectrum method in staad pro software to compare the results of storey displacement, column end forces and bending moments in beams for differentsoilconditions.Analysisresultsconveyedthefact thatonconsideringrealisticsupportconditionchangesthe column & beams forces in the structure. The relative displacements between successive floors are less for structureonsoftsoilssince,thestructureonsoftsoildeflects

as a whole body. Hence, the effect of soil structure interactioninsoftsoilsismoreascomparedtomediumand hard.

Finiteelementmodellingofanexistingundergroundwater tank has been done in ETABS software to understand the behaviour when subjected to seismic loading considering dynamic soil pressure & soil structure interaction using Winkler’sspringmethod.DubeyRahul etal[20] considered two different soil condition such as Clay of high Compressibility(CL)andSiltofhighCompressibility(MH) whereCLiscategorizedundermediumorstiffsoilandMHis undersoftsoiltocarryforwardtheanalysis.Hehighlighted thatalthoughsoilconditionsdonotinfluencedesignforces significantlybutonconsideringseismicforcesthemoments inwallsalongbothXandYdirectionsatthebaseexceeds moments in walls of existing tank. Further, shear force parameterisseendominatingthethicknessofwallbuton considering seismic forces, shear force increases which tends to redesign thickness of slab. Whereas, upon considering effect of soil structure interaction which is modelledwithelasticspringatbase,designforcesincrease comparedtoseismicdesignwithrigidbase.Although,effect ofsoilstructureinteractionisnotverymuchsignificant.

5. CODAL PROVISIONS ON SSI

Inspite of a vast array of solution techniques discussed in Section 4, very few international codes had given some guidelinesforincorporatingSoil StructureInteraction.This is generally due to lack of consent among researchers regardingtheeffectsofSoil StructureInteractionposedon seismicresponseofstructuresduringanalysis.Considering thesignificanceofconsideringSSIinstructuraldesign,there isaneedtoincludeSSIprovisionsinseismiccodesaround theglobe.Toenablethepeopleinvariouscodecommittees, this section makes a general attempt to discuss about the guidelinesregardingSSIinsomeoftheexistinginternational seismiccodes.

5.1 India

IS code 1893 3[21] & IS code 1893 part 4[22] spoken about considering Soil Structure Interaction while designing bridgesandindustrialstructures.Butit’salsonecessarythat Soil Structure Interaction needs to be considered while designingstructureswhichareboundtobesupportedupon deepfoundationsinsoftsoil.Thoughpastanalysesshows thatSoil StructureInteractionwouldleadtoreducedseismic forces further enhancing lateral deflections, still neither guidelines spoken about for computing Soil Structure Interaction effects nor specialist literature have been mentioned.Meanwhileseismiccodesforgeneralbuildings and liquid retaining structures, IS 1893 1[31] and IS 1893 2[23] arecompletelysilentaboutthephenomenon.

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5.2 United States of America

SSI provisions was first inducted by Applied Technology Council ATC 3 06[25] calling for a reduction in design base sheartocounterlongernaturalperiodandahigherdamping shownbystructure soilsystemascomparedtofixedbase. So,ASCE7 10[30] introducedacaponbaseshearreduction bysuggestingmodifieddesignbasesheartobenolessthan 70%oftheoriginalvalue.Ontheotherhand,reductionin equivalentlateralforcewasestablishedonelasticstructural response and further research shows that effect of SSI on structural response dwindles with the intensity of inelasticity experienced by the structure resulting in structural design not performing up to the desired mark duringseismicevent.NationalEarthquakeHazardReduction Program(NEHRP)developedsomeprovisionsintheformof FEMA[26] which basically put a limit on reduction of base shearasafunctionofresponsemodificationfactor.Hence, theseprovisionsrecommendreductionindesignbaseshear forsystemswithlargerresponsemodificationfactorwhich means those structures with larger inelastic deformation capacitywhichwerelaterincorporatedinASCE7 16[24]

Besides,theprocedureofequivalentlateralforce,ASCE7 16[24] suggestsalineardynamicanalysisusingeitherof thetwomethods

a) SSImodifiedgeneraldesignresponsespectrumas giveninthecode

b) SSImodifiedsite specificresponsespectrumwhich have to be developed by the concerned design engineer

Meanwhile, effects of kinematic interaction cannot be incorporated with the linear dynamic procedure. But if kinematic interaction is found to be predominant then, a nonlinear response history procedure using acceleration histories scaled to a site specific response spectrum for kinematicinteractionmethodshouldbeadopted.Kinematic SSI effects are represented by the response spectral modification factors for base slab averaging and with the productofembedmentnotlessthan0.7.Khosravikiaetal[32] evaluatedtheimportanceofusingFEMA[26]andASCE7 10[30] whichformthebasisofASCE7 16[24] andshowedthatSSI provisions of both FEMA[26] and ASCE 7 10[24] results in unsafe designs for structures with surface foundation on moderatelysoftsoils,butFEMA[26] slightlyimprovesupon thecurrentprovisionsbeingmoreconservativeoutofthe othertwoprovisionsspecified.

5.3 Japan

Dynamicinteractionbetweenthestructureandtheground shouldbetakenintoconsiderationwhiledesigningbridge abutments, retaining walls, underground structures and foundation structures such as piles and caissons as put forwardbyJSCE15[33].Butforotherstructures,SSIcanbe

ignoredorcanbemodelledappropriatelydependingonthe typeandcharacteristicsofstructureandground.Thechoice of modelling the structure and soil foundation system simultaneously or separately either by direct approach or substructureapproachislefttothejudgementofstructural designer.

5.4 Europe

EN1998 5[34]coderecommendsconsideringdynamicSSIfor structures which are either slender or have significant secondorder(��−��)effects.Thosestructuresfoundedeither on pile foundation or those with massive or deep seated foundationssuchasbridgepiers,offshorecaissonsandsilos alsopermitsincludingSSIintheirdesignprocess.EN1998 1[35] codespecificallymentionsa typical groundtypewith extremelylowshearstrengthandhighplasticityindexand EN1998 5[34] authorizesSSIconsiderationindesignofany structuretobebuiltonsuchdeposits.ThoughEN1998 5[34] identifiesthestructuresforwhichSSImustbeincludedin designpracticebutitdoesnotgiveanyspecificguidelinesto computetheeffectofSSI.

5.5 New Zealand

NZS 1170.5[36], similar to Eurocode, Indian Standard code and Japanese guidelines, did not designate any sort of guidelinestoincorporatetheeffectofSSIindesignpractice. However, it mentions the use of an entity termed as structural performancefactor whichdependsonmaterial, formandperiodoftheseismicresistingsystem,dampingof the structure, and interaction of the structure with the ground parameters. NZS 1170.5[36] also requires considerationoffoundationdeformationswhencalculating buildingdeflection.Butsincethereisnoguidanceonhowto include foundation flexibility, structural designers have frequently ignored foundation flexibility while designing basedonNZS1170.5[36] .

Amongthevariousstandardsdiscussedabove,itisseenthat ASCE 7 16[24] appears to be the most evolved while implementingtheeffectofSSIinstructuraldesignpractice. Moreover,itisalsoanoticeablefactthatmostofthewell knownseismiccodesdocontainconditionsforincludingSSI effects in design practice but still lack guidelines on evaluation and accommodation of SSI effects in design practice. Such gap accompanied by a communication gap betweenstructuralandgeotechnicalengineerscanmakea structuresusceptibletounsatisfactoryperformanceduring an earthquake. This highlights a greater importance of considering SSI in present day where good sites for construction are rare and construction on soft soils and landfillsarequitecommon.Inthiscontext,NIST[27]suggests proper guidelines and procedures to evaluate SSI effects whichcouldbeutilisedbetweenstructuralandgeotechnical engineersforcertainprojectmodifications.

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6. CONCLUSION

Thephenomenoncomprisingofvariousmechanismswhich leads to the interdependence of soil and structural displacementsisbasicallysoil structureinteractionwhichis broadly classified under either kinematic and inertial componentofsoil structureinteraction.Thepresentstudyis basedonreviewingpastresearchoverlastfewyearsonsoil structureinteractionandeffectofsoilflexibilityonresponse of structures. Soil Structure Interaction may prove to be either beneficial or detrimental to structural response during real seismic event contrast to the structure soil stiffness. But actual response is a function of frequency which depends on the earthquake intensity. It shows that rigid and heavy structures founded on soft soils are the worst hit. The response of any structure is analysed considering fundamental natural period, lateral displacement,storeydrift,lateraldeflectionandseismicbase shearasthemainparametershighlightingtheinfluenceof soil structure interaction on dynamic behaviour of the buildingandmustbeconsideredinthedesignofearthquake resistantbuildings.Itisalsorecommendedtoconsidersoil structure interaction analysis with increase of height of building and in case of soft soil to achieve accurate estimationforthedifferentstrainingactions

a) Buildingtotaldrift TheBaseflexibilitybehaviour, which is generated from the soil structure interactionanalysis,influencesthetotaldriftofthe building as it is clear through each storey displacement.So,itcanbeclearlymentionedthat fixed base or stiff soil assumptions can lead to greater underestimation of the storey drifts of building. Moreover, the study also confirms considerableeffectofbaseflexibilityontheinter storey drift ratios leading to superior serviceably limits.

b) Base shear and base moments Accordingly from the conducted study it is also confirmed that the buildingbaseshearandmomentsdiminisheswith mediumandsoftsoilsascomparedtostiffsoils.

So far, researchers who had analysed the interaction behaviourusingthelatestsoftwarepackagesconsideredthe foundation as raft foundation, isolated footing, pile foundation and mat foundation with the soil mass as homogenous, isotropic behaving as linear and nonlinear mannerintheinteractionanalysis.Forpracticalpurposes, Winklermodelshouldatleastbeemployed,assigningarea springsinthelocalzaxisoffiniteelementsoftwaretomake the foundation flexible there by creating the effect of soil structureinteractioninsteadofidealizingstructuresasfixed basetocarryouttheanalysis.

FromtheSSIprovisionslistedinsomeofthereputedseismic codesasdiscussedinsection4,itisseenthatASCE7 16[24]is

the only standard code with guidelines on implementing soil structureinteractionindesignandanalysisofstructures whereas other standard codes just suggest conditions for performing soil structure interaction analysis in design practicebutdonotestablishproperprocedureforthesame. Consideringstructuresoftenbeingconstructedonsoftsoils andlandfills,thisgapneeds tobefulfilledinformofwell drafted guidelines to ensure inclusion of soil structure interactioninregulardesignpracticeinthecomingfuture.A limited number of studies have been conducted so far consideringthesoilmassaselasto plastic,visco elasticand visco plastic in interaction analyses and effect of soil structureinteractiononbasementshearwallsofthebuilding due to various parameters which should be taken into considerationforcarryingoutresearchonSSIinthefuture. Also,transmissionandconsiderationofseismicwavesatthe interface between soil layers should be a further scope of studyincludingtheverticalwavepropagationanditseffect onbuildingresponse.Likewise,anotherfieldofstudywhich isSSSI(structure soil structureinteraction)andassociated phenomenaofstructuralpoundingisgainingpopularityin thecomingtimes.

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