Dalal Sejal P

INTERNATIONALJOURNALOFCIVILANDSTRUCTURALENGINEERINGVolume1,No 4,2011

Copyright2010AllrightsreservedIntegratedPublishingservicesResearcharticle ISSN0976 4399

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Performancebasedseismicdesignofstructure:AreviewDalalSejalP1 ,VasanwalaSA2,DesaiAK3

1 ResearchScholar,AppliedMechanicsDepartment,SVNIT,Surat,Gujarat,India.2 AssociateProfessor,AppliedMechanicsDepartment,SVNIT,Surat,Gujarat,India

3 Head,AppliedMechanicsDepartment,SVNIT,Surat,Gujarat,[email protected]

ABSTRACT

Presented in this paper is an updated literature review of the Performancebased Seismicdesign(PBSD)method.PerformancebasedSeismicdesignisanelasticdesignmethodologydone on the probable performance of the building under different ground motions. Thederivative of the PBSD method, known as the Performancebased Plastic design (PBPD)methodthathasbeenwidelyrecognizedasanidealmethodforuse inthefuturepracticeofseismic design has also been reviewed and discussed. Performancebased Plastic designmethodisadirectdesignmethodstartingfromtheprequantifiedperformanceobjectives,inwhichplasticdesign isperformed todetail the framemembersandconnections inorder toachievetheintendedyieldmechanismandbehavior.ThefindingsshowthatahugescopeofresearchworkisneededfordevelopmentofPBPDmethodforothertypeofstructures.

Keywords: Performance Based Seismic Design, Performance Based Plastic Design,Performanceobjectives,SeismicEvaluation.

1Introduction

Intherecentmajorearthquakes,itisnoticedthattheseismicriskinurbanareasisincreasingand the infrastructure facility is far fromsocioeconomically acceptable levels. There is anurgentneedtoreversethissituationanditisbelievedthatoneofthemostpromisingwaysofdoing this is through thePerformanceBasedEarthquakeEngineering (PBEE) inwhich thestructuraldesignisbasedonthepredictedperformanceofthestructureduringanearthquake.The PerformanceBased Earthquake Engineering (PBEE) also known as the PerformanceBasedSeismicEngineering(PBSE)isarapidlygrowingideathatispresentinallguidelinesthatwere recently published:Vision 2000 (SEAOC,1995),ATC40 (ATC,1996),FEMA273(FEMA,1997),andSAC/FEMA350(FEMA,2000a).

PBEE impliesdesign, evaluation, construction,monitoring the functionandmaintenanceofengineered facilitieswhose performance under seismic loads responds to the diverse needsand objectives of owners, users and society. In loose terms, it requires that a building bedesignedtomeetspecificperformanceobjectivesundertheactionofthefrequentortherarerseismiceventsthatitmayexperienceinitslifetime.So,abuildingwithalifetimeof50yearsmayberequiredtosustainnodamagesunderafrequent,50%in50yearsevent,e.g.,onethat has a probability of 50%of being exceeded in the next 50 years.At the same time itshouldbe able to remain repairable,despite sustainingsomedamage, during a 10% in 50years event and remain stable and lifesafe for rare eventsof 2% in 50 years, although,subsequently, itmayhavetobedemolished.Obviouslysuchperformanceobjectivescanbebettertailoredtoabuildingsfunction,e.g.,beingstricterforahospitalthatneedstoremainoperational even after severe events, while being more relaxed for less critical facilities,flexible and able to suit each building owners needs (respecting a minimum of safety of



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course).Ageneralmethodologywasformulatedinaneffort to involveall thevariablesthatmay affect the performance, such as seismic hazard, damage measures, collapse, financiallossesor lengthofdowntimeduetodamage,engineeringdemandssuchasstorydrifts,flooraccelerations, etc., (Krawinkler and Miranda, 2004). The performance evaluation of astructureiscarriedoutbyusingcomplexprobabilisticformulasandthedesignworkproceedsbygoingthroughseveraliterationsofthisprocess(Hamburger,2004).

The term PerformanceBased Seismic Design (PBSD) has been widely used by theengineeringandresearchcommunitysincethe1994NorthridgeEarthquake,perhapsthemostcostly earthquake in U.S. history, and other major earthquakes around the world whichoccurred at the end of the 20th century. This PBSD of buildings has been practiced sinceearlyinthetwentiethcentury,England,NewZealand,andAustraliahadperformancebasedbuilding codes in place for decades. The International Code Council (ICC) in the UnitedStateshadaperformancecodeavailableforvoluntaryadoptionsince2001(ICC,2001).TheInterJurisdictional Regulatory Collaboration Committee (IRCC) is an international grouprepresenting the lead building regulatory organizations of 10 countries formed to facilitateinternationaldiscussionofperformancebasedregulatorysystemswithafocusonidentifyingpublic policies, regulatory infrastructure, education, and technology issues related toimplementingandmanagingthesesystems.

In1989, the FEMAfundedprojectwas launched todevelop formal engineering guidelinesforretrofitofexistingbuildingsstarted,ATC,1989),itwasrecommendedthattherulesandguidelines be sufficiently flexible to accommodate a much wider variety of local or evenbuildingspecific seismic risk reduction policies than has been traditional for new buildingconstruction.Theinitialdesigndocument,NEHRPGuidelinesfortheSeismicRehabilitationof Existing Buildings, FEMA 273, therefore contained a range of formal performanceobjectives thatcorresponded to specified levelsof seismic shaking.Theperformance levelswere generalized with descriptions of overall damage states with titles of Operational,ImmediateOccupancy,LifeSafety,andCollapsePrevention.These levelswere intendedtoidentifylimitingperformancestatesimportanttoabroadrangeofstakeholdersbymeasuring:theabilitytousethebuildingaftertheeventthetraditionalprotectionoflifesafetyprovidedby building codes and, in the worst case, the avoidance of collapse. Following theNorthridge event, the Structural Engineers Association of California (SEAOC, 1995)developed a PBSDprocess, known asVision 2000,whichwasmore generalized than thatcontainedinFEMA273butusedsimilarlydefinedperformanceobjectives.

Over the 10year period afterpublication ofFEMA273, its procedureswere reviewed andrefinedandeventuallypublishedin2006asanAmericanSocietyofCivilEngineers(ASCE)national standard Seismic Rehabilitation of Existing Buildings, ASCE 41. Althoughintended for rehabilitation of existing buildings, the performance objectives andaccompanyingtechnicaldatain ASCE41respondedtothegeneralinterestinPBSDandhavebeen used for the design of newbuildings to achieve higher ormore reliable performanceobjectivesthanperceivedavailablefromprescriptivecodeprovisions.ASCE41isconsideredtorepresentthefirstgenerationofperformancebasedseismicdesignprocedures.



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2.ApplicationsandadvantagesofthePBSDmethod

In contrast to prescriptive design approaches, performancebased design provides asystematicmethodologyforassessingtheperformancecapabilityofabuilding.Itcanbeusedto verify the equivalent performance of alternatives, deliver standard performance at areducedcost,orconfirmhigherperformanceneededforcriticalfacilities. Italsoestablishesavocabulary that facilitates meaningful discussion between stakeholders and designprofessionalsonthedevelopmentandselectionofdesignoptions.Itprovidesaframeworkfordetermining what level of safety and what level of property protection, at what cost, areacceptable to stakeholders based upon the specific needs of a project. Performancebasedseismicdesigncanbeusedto:

Design individual buildingswith a higher level of confidence that the performanceintendedbypresentbuildingcodeswillbeachieved.

Designindividualbuildingsthatarecapableofmeetingtheperformanceintendedbypresentbuildingcodes,butwithlowerconstructioncosts.

Design individual buildings to achieve higher performance (and lower potentiallosses)thanintendedbypresentbuildingcodes.

Assessthepotentialseismicperformanceofexistingstructuresandestimatepotentiallossesintheeventofaseismicevent.

Assess the potential performance ofcurrent prescriptivecode requirements for newbuildings, and serve as the basis for improvements to codebased seismic designcriteriasothatfuturebuildingscanperformmoreconsistentlyandreliably.

Performancebasedseismicdesign isbothefficientandeffective toavoid futureearthquakelosses. Further, the technology used to implement performancebased seismic design istransferable, and can be adapted for use in performancebased design for other extremehazardsincludingfire,wind,flood,snow,blast,andterroristattack.

TheadvantagesofPBSDoverthemethodologiesusedinthecurrentseismicdesigncodearesummarizedasbelow

1.Multilevel seismic hazards are consideredwith an emphasis on the transparency ofperformanceobjectives.2.Buildingperformanceisguaranteedthroughlimitedinelasticdeformationinaddition tostrengthandductility.3. Seismic design is oriented by performance objectives interpreted by engineeringparametersasperformancecriteria.4.Ananalyticalmethodthroughwhichthestructuralbehavior,particularlythenonlinearbehaviorisrationallyobtained.5. The buildingwillmeet the prescribedperformance objectives reliablywith acceptedconfidence.6.Thedesignwillensuretheminimumlifecyclecost.

3.Designprocedure ofthePBSDmethodandevolutionofthePBPDmethod

Performancebaseddesign beginswith the selectionofdesigncriteria stated in the formofone or more performance objectives. Once the performance objectives are set, a series of



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simulations(analysesofbuildingresponsetoloading)areperformedtoestimatetheprobableperformanceof thebuildingundervariousdesignevents. Inthecaseofextremeloading,aswouldbe imparted by a severe earthquake, simulationsmay be performed using nonlinearanalysis techniques. If the simulated performance meets or exceeds the performanceobjectives,thedesigniscomplete.Ifnot,thedesignisrevisedinaniterativeprocessuntiltheperformance objectives are met. In some cases it may not be possible to meet the statedobjectiveatreasonablecost,inwhichcase,somerelaxationoftheoriginalobjectivesmaybeappropriate.

After theconceptualdesignphaseiscompleted, thenumericaldesignphase isproceededtodetermine the structural detailing, which satisfy the prequantified performance objectives.Preliminary designcanbeconductedthroughtwodifferentapproaches:

1. Traditional forcebased design method followed by the check of performanceobjectivesand

2. Directdesignmethodstartingfromtheprequantifiedperformanceobjectives.

Theresultsobtainedby thedirectdesignmethodarebelievedtobeclosertothefinaldesignandrequirelesscomputationaleffort.Thus,thisfindingleadstoevolutionofthePerformanceBasedPlasticDesign(PBPD)methodwhichisadirectdesignmethodthatusespreselectedtarget drift and yieldmechanismsas key performance objectives that determine the degreeanddistributionofexpectedstructuraldamage.ItisbasedontheformulationsderivedfromthecapacityspectrummethodusingNewmarkHall reduction factors (NewmarkandHall,1982)fortheinelasticdemandspectrum.

Figure1: ProcurementoftheTargetDrift

Thedesignbaseshearforaspecifiedhazardleveliscalculatedbyequatingtheworkneededto push the structure monotonically up to the target drift to the energy required by anequivalent EPSDOF to achieve the same state. Also, a new distribution of lateral designforcesisusedthatisbasedonrelativedistributionofmaximumstoreyshearsconsistentwith

TargetDrift

DemandCurve

CapacityCurve

D

A



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inelasticdynamicresponseresults(Chaoetal.,2007).Plasticdesignisperformedtodetailthe framemembers and connections in order toachieve the intended yieldmechanism andbehavior.This targetdrift isobtainedby intersecting thedemanddiagramand thecapacitydiagramof the structureas shown in figure1.And theYieldmechanism ischosen tobeaplastichingeformationinthebeamsandatthebasecolumnasshowninfigure2.

Figure2:TheFormationofPlasticHingesfortheyieldmechanism

Results of extensive inelastic static and dynamic analyses have proven the validity of themethod (Goel and Leelataviwat, 2008). The method Comparisons of responses withcorresponding baseline frames designed by current practice have consistently shownsuperiority of the proposed methodology in terms of achieving the desired behavior. Themethod isespeciallyadvantageousfor tall frames,wherecumbersomeand lengthy iterativedesign work in current practice can be completely eliminated, while leading to excellentperformanceastargeted.

Since the numerical phase of performancebased design is an iterative procedure betweendesignandverification, inordertosavecomputationaleffort, it issuggestedtoselectfewerperformanceobjectivesinthepreliminarydesignandcheckallperformanceobjectivesinthefinal design. The decision as to howmany and which performance objectives need to beselecteddependsonifthatperformanceobjectiveisthemainconcernoftheusersandownersandifquantificationoftheperformanceacceptablelimitisreliable.

PBPDdesignhasbeensuccessfullyappliedtoSteelMomentResistingFrame(LeeandGoel,2001),bucklingrestrainedbracedframe,EccentricallyBracedFrame(ChaoandGoel,2006),SpecialTrussMomentFrame (Chao andGoel , 2008), concentric braced frames(Chao andGoel ,2006)andcompositebuckling restrainedbraced frame(Dasguptaetal,2004). Inallcases, the frames developed the desired strong columnweak beam yield mechanisms asintended,andthestoreydrifts/ductilitydemandswerewithintheselecteddesignvalues,thusmeetingtheselectedperformanceobjectives(Goeletal,2010).AdraftcodeforTaiwanhasalsobeenrecentlydevelopedbyXue(Xueetal,2008)basedonthePBPDmethodofdesignforimplementation.

PBPDdesign has also been applied toReinforcedConcrete SpecialMomentStructures byLiao(LiaoandGoel,2010).DevelopmentofthePBPDmethodologyforReinforcedConcrete



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structures with degrading hysteretic behavior is currently in progress. Seismic design ofReinforced Concrete structures to achieve targeted response presents special challengemainlyduetotheircomplexhystereticbehavior.Thisstudyisprimarilyanalytical innatureand focusesonReinforcedConcretemoment frames. It is expected that findings from thisstudy will be incorporated in the next generation of performancebased design codes andpractice.

4.SummaryandDiscussion

Several approaches for the PBSD method proposed by researchers have been brieflyreviewed in this paper and it is observed thatmore researchwork isneeded especially fordevelopmentofPBPDmethodforvariousotherdifferent typesofstructures.It isimportanttonotethatinthePBPDmethod,controlofdriftandyieldingisbuiltintothedesignprocessfromtheverystart,eliminatingorminimizingtheneedforlengthyiterationstoarriveatthefinal design. Other advantages include the fact that innovative structural schemes can bedeveloped by selecting suitable yielding members and/or devices and placing them atstrategic locations, while the designated nonyielding members can be detailed for no orminimumductilitycapacity.Allof thesewouldtranslate intoenhancedperformance,safetyandeconomyinlifecyclecosts.AsthePBPDacceptsdamageinseismicevents,andprovestobethemosteconomicalsolution,andtheperformancecanbequantifiedandconfirmedtothe owners desires, it is quite possible that it can be misused by the owner for personalprofits.

ListofAbbreviations

ASCE AmericanSocietyofCivilEngineersATC AppliedTechnologyCouncilEPSDOF ElastoPlasticSingleDegreeofFreedomFEMA FederalEmergencyManagementAgencyICBO InternationalConferenceofBuildingOfficialsICC InternationalCodeCouncilNEHRP NationalEarthquakeHazardReductionProgrammePBEE PerformanceBasedEarthquakeEngineeringPBPD PerformanceBasedPlasticDesignPBSD PerformancebasedSeismicdesignSEAOC StructuralEngineersAssociationofCaliforniaUBC UniformBuildingCode

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