ASCE_7-05_Seismic_Provisions

ASCE7-05SeismicProvisionsABeginner’sGuidetoASCE7-05Dr.T.BartQuimby,P.E.,F.ASCEQuimby&AssociatesHighprobabilityof“Failure”“Failure”redefinedtopermitbehavior(yielding)thatwouldbeconsideredfailureunderotherloads.HighUncertaintyImportanceofDetails“Indealingwithearthquakeswemustcontendwithappreciableprobabilitiesthatfailurewilloccurinthenearfuture.Otherwise,allthewealthoftheworldwouldproveinsufficient…Wemustalsofaceuncertaintyonalargescale…Inaway,earthquakeengineeringisacartoon…Earthquakessystematicallybringoutthemistakesmadeindesignandconstruction,eventheminutestmistakes.”Newmark&Rosenblueth2ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05HazardLevelsIncipientCollapseLifeSafetyImmediateReoccupancyFullyOperationalOccasional50%in50yearsRare10%in50yearsVeryRare5%in50yearsMaxConsidered2%in50yearsPerformanceLevels3ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05DesignObjectiveDefinedAspecificperformancelevelgivenaspecificearthquakehazardlevelStatedbasisofcurrentcodes:Lifesafety(+somedamagecontrol)at10%in50yearevent(nominally)4ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05PurposeoftheProvisionsFEMA302Section1.1“Thedesignearthquakegroundmotionlevelsspecifiedhereincouldresultinbothstructuralandnonstructuraldamage.FormoststructuresdesignedandconstructedaccordingtotheseProvisions,structuraldamagefromthedesignearthquakegroundmotionwouldberepairablealthoughperhapsnoteconomicallyso.Foressentialfacilities,itisexpectedthatthedamagefromthedesignearthquakegroundmotionwouldnotbesosevereastoprecludecontinuedoccupancyandfunctionofthefacility.”“Forgroundmotionslargerthanthedesignlevels,theintentoftheseProvisionsisthattherebealowlikelihoodofstructuralcollapse.”5ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05CompareWindandSeismicDesignofSimpleBuilding120’90’62.5’Earthquake:Assume0.4gNEHRPWind:100MPHExposureCBuildingProperties:MomentResistingFramesdensityr=8pcfPeriodT=1.0secDampingx=5%4.36ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05Wind:120’90’62.5’100mphFastestmileExposureCVelocitypressureqs=25.6psfGust/ExposurefactorCe=1.25PressurecoefficientCq=1.3LoadFactorforWind=1.3Totalwindforceon120’face:VW120=62.5*120*25.6*1.25*1.3*1.3/1000=406kipsTotalwindforceon90’face:VW90=62.5*90*25.6*1.25*1.3*1.3/1000=304kips4.47ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05Earthquake:120’90’62.5’BuildingWeightW=120*90*62.5*8/1000=5400kipsTotalELASTICearthquakeforce(ineachdirection):VEQ=0.480*5400=2592kipsCASTSV121204101004802323......//VCWEQSThisexampleusesanoldversionofboththeNEHRPandtheASCE7WindLoadCriteria.Itisusedforillustrativepurposesonly.8ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05Comparison:Earthquakevs.WindVVEQW120295240673.VVEQW90295230497.•ELASTICEarthquakeforcesare7to10timeswind!•Virtuallyimpossibletoobtaineconomicaldesign4.69ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05HowtoDealwithHugeEarthquakeForce?•Isolatestructurefromground(BaseIsolation)•IncreaseDamping(PassiveEnergyDissipation)•AllowInelasticResponseHistorically,BuildingCodesuseInelasticResponseProcedure.Inelasticresponseoccursthoughstructuraldamage(yielding).Wemustcontrolthedamageforthemethodtobesuccessful.4.710ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05InterimConclusion(TheGoodNews)Theframe,designedforawindforcewhichis15%oftheELASTICearthquakeforce,cansurvivetheearthquakeif:IthasthecapabilitytoundergonumerouscyclesofINELASICdeformationItsuffersnoappreciablelossofstrengthIthasthecapabilitytodeformatleast5to6timestheyielddeformationREQUIRESADEQUATEDETAILING4.1211ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05InterimConclusion(TheBadNews)Asaresultofthelargedisplacementsassociatedwiththeinelasticdeformations,thestructurewillsufferconsiderablestructuralandnonstructuraldamage.Thisdamagemustbecontrolledbyadequatedetailingandbylimitingstructuraldeformations(drift)4.1312ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05Elasticvs.InelasticResponseTheredlineshowstheforceanddisplacementthatwouldbereachedifthestructurerespondedelastically.Thegreenlineshowstheactualforcevs.displacementresponseofthestructureThepinklineindicatestheminimumstrengthrequiredtoholdeverythingtogetherduringinelasticbehaviorThebluelineistheforcelevelthatwedesignfor.Werelyontheductilityofthesystemtopreventcollapse.From1997NEHRPProvisions13ASCE7-05SeismicProvisions-ABeginner'sGuidetoASCE7-05HistoricalDevelopmentofSeismicCodes1755-Lisbon:groundshakingwaves1906-SanFrancisco:Fire,lateralforcefromwind1911-Messina,Italy:Staticinertialforce(10%),FirstrecognitionofF=ma1923-Tokyo:Predictionbyseismicgap1925-SantaBarbara:USCGSinstructedtodevelopstrongmotionseismographs.1927-U.B.C.:InertialforcesandsoileffectsintheU.S.(7.5%or10%ofD+L)1933-LongBeach:Firstinstrumentalrecords(flawed):reinforcementrequiredformasonry;qualityassurance;designreview&constructioninspection.14ASCE7-05SeismicProvisions-ABeginner'sGu