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Earthquakeresistantbuildingdesigncodesandsafetystandards:TheCaliforniaexperience1

StephenH.Cutcliffe

HistoryDepartment,LehighUniversity,9W.PackerAvenue,Bethlehem,PA18015-3081U.S.A.(Tel:610-758-3350;Fax:610-758-65)

Received26June2000;accepted20December2000

Keywords:buildingdesigncodes,earthquakeengineering,EarthquakeEngineeringResearchInstitute,Northridge,Calif.earthquake(1994),SanFernando,Calif.earthquake(1971),SanFranciscoearthquake(1906),seismology,StructuralEngineersAssociationofCaliforniaAbstract

Seismologistsandearthquakeengineershavesoughttounderstandandpredictearthquakesandtodevelopbetterbuildingdesignstowithstandthemforwelloveracentury.IntheUnitedStates,the1906SanFranciscoearthquakeprovidedthefirstrealimpetusforestablishingbuildingdesigncodesandsafetystandards.SubsequentmajorCaliforniaearthquakesinSantaBarbara(1925),LongBeach(1933),SanFernando(1971),LomaPrieta(19),andNorthridge(1994)eachledtoadditionalseismologicalunderstandingandengineeringresponseintheformofenhancedbuildingdesigncodes.Nonetheless,theprocesstoincorporategoodseismicdesignandmitigationeffortshasbeenslow,andbynomeansfailsafe,especiallyintheEasternU.S.wheremuchofthebuildingstockpredatesmorerecentdesigncodes,andhencewhereamajorearthquakecouldcollapselargenumbersofbuildings.Evenintheabsenceofcatastrophe,itisstillimportanttoguardagainstafalsesenseofsecurity.

Earthquakesareamongthemostdangerousandde-structiveofnaturaloccurrencesasthe1994earthquakeinNorthridge,Calif.andthatof1995inKobe,Japan,aswellasothersinIran,Afghanistan,andTaiwan,remindedtheworld.2However,attemptstounderstandandpredictearth-quakesandtodevelopbetterdesignsforbuildingsinordertowithstandearthquakeshaveoccupiedtheattentionofseis-mologistsandearthquakeengineersforwelloveracentury.Forexample,inthemid-tolatenineteenthcentury,BritishandJapaneseseismologistsandengineerscontributedtothedesignofseismicinstrumentationandfoundedtheSeismo-logicalSocietyofJapan,oneofthefirstofmanysuchorga-nizationsthattodaymakeuptheInternationalAssociationforEarthquakeEngineering(Housner,1990).

IntheUnitedStatesitwasthe1906SanFranciscoearth-quakethatprovidedearthquakeengineeringandseismicdesignitsfirstrealimpetus.Thelossofapproximately1,000livesanddamageestimatesofbetweenhalftoonebilliondollars,muchofitduetoresultingfires,devastatedthecity.Therelativelysmallnumberofsteelframebuildings,whiledamaged,generallysurvived.TheearthquakealsoledtotheestablishmentoftheStructuralAssociationofSanFran-cisco,latertobecometheStructuralEngineersAssociationofCalifornia(SEAOC),andthecreationoftheSeismologi-calSocietyofAmerica.Duetotheparticularlyearthquake-pronenatureofCalifornia,itwouldbeoutoftheSEAOC’srecommendationsthatmostsubsequentearthquake-resistantbuildingcodeprovisionswouldemerge.Subsequentearth-quakesprovidedadditionaldestructiveobjectlessons,andoutofthemcameaseriesofevermorestringentbuildingcoderequirements.3

EarthquakesinSantaBarbarain1925andinLongBeachin1933,bothofwhichmeasured6.3ontheRichterscale,re-inforcedtheneedforbetterseismologicalunderstandingandimprovedbuildingdesigns.4Thedevelopmentofthefirststrong-motionaccelerographsintheearly1930sallowedearthquakeengineerstoactually‘see’thenatureofexten-sivegroundshakingintermsofamplitude,frequency,andduration.Thesetwoearthquakesalsorevealedthatsomeofthelargestdamageproblemswerewithunreinforcedbrickbuildingsconstructedwithsand-limemortar,atypeofcon-structionthentypicalofmanyschoolbuildings.WithinamonththeCaliforniaLegislaturepassedtheFieldAct,whichrequiredenhancedlateralforcedesignstandardsforallsubsequentschools.Thisacthasprovenitselftobeex-tremelyimportant,forduringsubsequentearthquakes,manypre-1933schoolssufferedseveredamage,whilethosesub-sequentlyerectedormodifiedaccordinglyfaredfarbetter.Althoughsomewhatweakerintermsofstandards,the1933RileyActincorporatedsimilarrequirementsfornon-schoolbuildingsandhassimilarlyprovenitsworth.AtthesametimeindividualcitiessuchasLosAngelesbeganincorpo-ratingmandatory,ratherthanmerelysuggestive,earthquakeprovisionsintotheirbuildingcodes.5

Theperiodfromthemid-1930sthroughthe1960swasmarkedbythesteadyaccumulationofseismologicaldata

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fromaseriesofCaliforniaearthquakesandthelarger19Alaskaearthquake.6Thedatahelpedearthquakeengineersrecognizetheneedtoincorporatedifferentdesigncodesforvaryingbuildingflexibilities,fornobuildingisentirelyrigid.Thisdynamicunderstandingwasreflectedinthe1943LosAngelesbuildingcodeandagainimproveduponin1957atthetimeathirteen-storyheightlimitationforthecitywasremoved.TheSEAOCformedaSeismologyCommit-teein1957whoseresulting\"RecommendedLateralForceRequirements\"reporthassubsequentlybeenadoptedbynu-merousbuildingcodebodiesaroundtheworld,includingbytheUniformBuildingCodeintheU.S.in1961.Thephilosophythatunderliestherequirementsisnotoneofab-solute100%,risk-freeprotection,whichisneverpossible,butratherthe‘reasonable’protectionoflifeandproperty,whichincludesformajorearthquakesresistancetocollapse,butrecognizesthelikelihood7of‘somestructuralaswellasnonstructuraldamage.’In1949theEarthquakeEngi-neeringResearchInstitute(EERI)wasfounded,andtodayisanorganizationofover1600members.Itsactivitiesin-cludeconductingpost-earthquakereconnaissancesurveys,apublishingprogram,andthehostingofregularconferences.AnotherimportantorganizationistheInternationalAssoci-ationforEarthquakeEngineering,whichholdstheWorldConferenceonEarthquakeEngineeringeveryfouryears.Likeotherareasofengineering,oneoftenlearnsthemostbystudyingactualfailures.

The1971SanFernandoearthquake8reallyadvancedtheunderstandingofaseismicdesign.Althoughofonlyamod-est6.4magnitude,itresultedinsixty-fivedeathsandover$550millioninpropertydamage.Forty-sevenofthedeathsresultedfromthecollapseofseveralpre-1933unreinforcedmasonryhospitalbuildings,whichledtopassagethefollow-ingyearoftheCaliforniaHospitalSeismicSafetyAct.ThisactshiftedconstructionplanningandresponsibilityfromthelocaltostatelevelthroughtheestablishmentoftheOfficeofStatewideHealthPlanningandDevelopment.AnotherimportantlessondrawnfromtheSanFernandoearthquakewastheimportanceofnon-structuralorarchitecturaldam-age,whichaccountedforover50%ofthetotaldamage,andincludedsuchbuildingcomponentsaselevatorsys-tems,ceilings,electrical,plumbing,andventilatingsystems,andinadequatelysecuredstorageequipment.Oneadditionalareaofconcernalsoreceivingincreasedattentionwaswhathasbecometermed‘lifeline’systems.Suchsystemsincludeenergy,water,transportation,andcommunication,failureofwhichentailsnotonlyfinancialloss,butmaycontributetohealthandsafetyconsiderations.

TheSEAOC,promptedbytheSanFernandoquake,in1974subsequentlyreviseditsseismiccoderecommenda-tions,whichwereinturnincorporatedintothe1976editionoftheUBC.9Includedintheenhancedrequirementswereconsiderationsforthe‘seismicity’oftheareainwhichabuildingwaslocated,theprimaryusageoroccupancyofabuilding,andafactorthataccountsfordifferencesinunder-lyingsoils.Eachofthesenewlyincorporatedfactorsentailedhigherlateralforcerequirementsfortheincreasedrisksasso-ciatedwiththeearthquakepronenessofanarea,thedangers

ofunderlyingsoils,andthehighersafetylevelsneededforsomestructuresoverothers.Finally,theincreasedavailabil-ityofcomputersandtheirassociatedmodelingcapabilitiesalloweddesignerstoincorporateandcodewriterstorequiredynamic,ratherthanstaticanalysisofnon-uniformbuild-ingsofirregularshapethatentaildifferencesinstiffnessbetweenadjacentsections.Thisdevelopmentofcomputermodelingtechniquesmadeitpossibletobuildhigh-risebuildingsinparticularlyearthquake-pronezones,suchasinLosAngelesandinJapan.Thelattercompleteditsfirstrealskyscrapersonlyinthelate1960s,includingthe40-storyTokyoWorldTradeCenterBuildingin1970,andthemorerecentlycompleted45-storyTokyoCityHallBuild-ingin1995(Berlin,1980,Vol.2,pp.88–91;Green,1987,pp.31–32;Muto,1971).

The‘moderate’6.9LomaPrieta,Californiaearthquake,whichdisruptedthe3rdgameofthe19WorldSeriesinSanFrancisco,resultedinoneofthelargestnumberofdeathsever(63)anduptothatpointcausedthemostprop-ertydamage($7.1billion)fromaU.S.earthquake.10Amongthemostvulnerablebuildingswerethosewith‘weak’firstfloors,suchasthreetofourstoryresidenceswithnumer-ousgaragedooropeningsatgroundlevel,whichledtosignificantdamage,especiallywhereextensiveliquefactionofthesoiloccurred(Fratessa,1994).Thisproblemhasledtoincreasedattentionbeingpaidtothedevelopmentofsoilliquefactionmaps,and,insomecasesatleast,en-hancedlanduseplanningdecisionsmadebasedonsuchmaps(Cloughetal.,1994).Anotherimportantlessonfromthisearthquakeconcernedincreasedattentiontothelossofbuildingfunctionalityandtheimpactthiscouldhaveonlocalcommunities(Fratessa,1994;Tobin,1994).

LessthanfiveyearslateronJanuary17,1994themostdestructiveU.S.earthquaketodatestruckNorthridge,Cal-ifornia.At6.7ontheRichterscale,theNorthridgeearth-quakewasslightlysmallerthanthatwhichhadsodisruptedthe19WorldSeries.Yet,becauseofstronggroundmotionitwouldprovetobethemostdestructiveU.S.earthquakeeverintermsofpropertydamagewithlossesamountingtoover$20billion.Almostmiraculously,only57peoplelosttheirlives.Whatwasmostsurprising,atleastfromanen-gineeringperspective,wasthedamagedonetostructuresbuiltaccordingtorecentdesigncodesandthoughttobeearthquakeresistant.11

Seismicrecordingsrevealedverydramaticandrapidgroundmovement,bothlaterallyandvertically,thanpre-viouslyanticipated–withsomegroundmovementsontheorderof2ft(0.61m)andthenbackagainwithinaperiodofbut5sec.Suchmovementsarefarbeyondthelimitsan-ticipatedinexistingbuildingcodes,andsuggestthatatallstructure,evenifbuilttothemoststringentcodes,mightnotbeabletohandlesuchmovementandstress.NorthridgeitselfthankfullyhadfewtallbuildingorstructuresolderthantwentyyearsincontrasttoKobe,Japan,alarger,morebuiltupcity,whichborethedirectbruntofitsearthquakeexactlyoneyearlater.HadLosAngelesitselfbeenattheepicenter,itisconceivablethatanynumberoflargeskyscrapersmightwellhavefallen.

TheNorthridgequakerevealedanumberofdesignprob-lems,oneofthemoreseriousbeingthree-story,wooden-framedapartmentbuildingsframedinwoodandsheathedwithgypsumboardorstucco.Hundredsofblocksofsuchbuildingscollapsedorsufferedextensivedamage.Hereagain,aswithLomaPrieta,oneofthecontributingfactorswasthelargenumberofgaragesandopencarportsunder-neathwhichweakenedthestructures’abilitytoresistlateralforces.Accordingtooneestimate,approximatelyhalfoftheprojected$20billionintotaldamageswasduetofailuresoforproblemswithwooden-framedbuildings.Muchofthisdamagecanbeattributedtopoorconstructionpracticeandinadequatefieldinspectionaswellasunsounddesign.Thus,amongtheproposalstoemergefromthepost-earthquakeanalysiswereproposalsformoreextensiveandadequateinspectionprocedures(Halletal.,1995).

Ofallthedamage,however,themostdisconcertingwasthefailureofweldedsteelmoment-frame(WSMF)structurespreviouslythoughttobeverysafeseismically.Subsequentexaminationofsuchbuildingshasrevealedthatmorethan120hadcracksintheconnectingjointswherethebeamsareweldedtothesupportingcolumns.Whilenonefailedcatastrophically,itwasanopenquestionwhethertheycouldsurviveanothersuchearthquake.Theproblemiscom-poundedbythedifficultyofdetectingsuchdamageduetothefactthatthestructuralsteelisoftencoveredbyarchitec-turalfinishingandfireproofingmaterials.TheoverallresultisthatL.A.hasnowtightenedupitsinspectionofsuchbuild-ings,withtherequiredrepairsbeingexpectedtocostsome$300million.Oneoftheespeciallytroublingaspectsofthisdiscoveryisthatthisparticulardesignwasusedinmorethanhalfofallnewcommercialbuildingserectedworldwidein1994.12

Animportantlessontobelearnedfromallearthquakes,butespeciallyfromtheunexpecteddamageoccasionedbytheNorthridgeearthquake,isthatwemustnotbecomecomplacentaboutearthquakeengineeringandtheaseismicdesignofbuildings.ThislessonisnolessapplicableoutsideCaliforniawheremostoftheattentionhasbeenfocused.IfinCaliforniatheefforttoincorporategoodseismicdesignandenhancemitigationeffortshasbeenlongandslow,theeffortelsewhere,especiallyintheEasternU.S.,hasbeenevenmoreuphill.There,althoughlessseismicallyprone,mostofthehigh-risebuildingisalreadyinplace,andamajorearthquakecouldcollapselargenumbersofbuildings.Thepriorityofotherpublicissues,theimmediacyofcostscoupledwithuncertainbenefits,andtheinvisibilityofpublicsafety,atleastintheabsenceofcatastrophe,allworkagainsttheadoptionandenforcementofseismicbuildingcodes.13Allearthquakeprofessionalsrealizeitisonlyamatteroftimebeforethenextquakewilloccur,andthatitisanissueoflocationandmagnitudethatwilldeterminethelossoflifeandtheextentofdamage.Thus,wemustguardagainstafalsesenseofsecurity.

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Acknowledgements

Theessaywasoriginallypresentedatthe25thSymposiumoftheInternationalCommitteefortheHistoryofTechnol-ogy,August20,1998,Lisbon,PortugalaspartofasessiondevotedtoTechnologyandNaturalDisasters.IwouldliketothankJamesWilliamsforinvitingmetoparticipateinthatmeetingandforhishelpfulcommentswithregardtopreparingthispaper.Additionally,IamindebtedtoKeiichiShimizuforsuggestingIpursuethisstudyaspartofalargerprojectsupportedbytheJapaneseMinistryofEducationtowhichorganizationIalsowanttoexpressmyappreciation.

Notes

1Alongerandmoredetailedanalysisoftheissuesdiscussedinthis

paper2maybefoundinCutcliffe(1996).

Aninsightfulhistoricalanalysisofearthquakeengineering,especiallyforthepre-WorldWarIIperiodisWilliams(1995),butalsovaluableforthepost-wareraisBerlin(1980).ArecentsummaryofearthquakeengineeringthatdrawsontheNorthridgeandKobedisastersisEarthquakeEngineering(1995).

3ForasummaryoftheSanFranciscoearthquakeanditsimplicationsforthebuildingengineeringresponse,seeWilliams(1995,pp.5–12);Berlin(1980,Vol.2,pp.77–79);Popov(1988,pp.817–118);andSteinbrugge(1970,4pp.174–181).

ForsummariesoftheSantaBarbaraandLongBeachearthquakesandtheengineeringresponsessubsequentlyengendered,seeDewellandWillis(1925);Report(1925);Freeman(1932,Ch.16,passim,pp.694–698,724–734);Berlin(1980,Vol.2,pp.78–80);Steinbrugge(1970,pp.181–183);andWilliams5(1995,pp.12–14,16–18).

OntheFieldandRileyActs,seeMeehan(1973,Vol.1,Pt.B,pp.667–670);Steinbrugge(1970,pp.183–187);Berlin(1980,Vol.2,pp.80–81,94–96);Dewell(1939,pp.603–604);andMoranandBockemohle(1973,p.24).

6Foranextendeddiscussionofthisperiod,seeCutcliffe(1996),butalsoBerlin(1980,Vol.2,pp.80–81,109–117)andGreen(1987,pp.19–24,27–28,732).

SeeBerlin(1980,Vol.2,p.81)andEERI(1994)fordetailsoftheSEAOC8lateralforceprinciples.

DetailsregardingtheSanFernandoearthquakecanbefoundinBenfer(1973)9andBerlin(1980,Vol.2,pp.118–133,142–156).

Fordetailsonthe1975SEAOCCode,seeGreen(1987,pp.27-32),whowashimselfamemberofboththeoriginal1957andthe1970AdHocSEAOCcommittees.The1975SEAOCCodeasreflectedinthe1976UBCEarthquakeRegulationsisreproducedinBerlin(1980,AppendixD,Vol.3,pp.173–183).10SeealsoMoranandBockemohle(1973,pp.29–31).

ExtensivecoverageoftheLomaPrietaearthquakecanbefoundinTheNewYorkTimes,October18–20,19.SeealsoNationalResearchCouncil11(1994).

TheNewYorkTimesprovidedextensivecoverageoftheNorthridgeearthquakeinthedaysfollowingtheeventasthestoryunfolded.SeeEarthquakeEngineering(1995,pp.3–4)forabriefbutmoreanalyticalassessment.Formoreextendedanalyses,seeHall(1995)andHolmesandSomers12(1996).

Forfurtherdetailsontheproblemsassociatedwithsteelframedbuildings,seeFritsch(1994);Krawinkleretal.,(1996);andEarthquakeEngineering13(1995).

ForinformationonearthquakemitigationoutsideCalifornia,seeOl-shansky(1994);Showalter(1994);BerkeandBeatly(1992);andChang(2000).

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