关于乌什桥在台湾921大地震中破坏情况的研究外文翻译.docx

关于乌什桥在台湾921大地震中破坏情况的研究外文翻译.docx

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关于乌什桥在台湾921大地震中破坏情况的研究外文翻译

StudyofDamagedWushiBridgeinTaiwan921Earthquake

ByYaoT.Hsu1andChungC.Fu2,Members

Abstract:

ThispaperreportsonthedamageofWushibridgeinarecentTaiwan921earthquake.DamagetoWushibridgeappearedinthesuperstructure,thesubstructureandtheapproaches.Typicaltypesofdamagearediscussedandillustratedinthispaper.AreviewofthebridgedesignspecificationsinTaiwanisalsopresentedtogivethebackgroundontheseismicdesignofTaiwanhighwaybridges.

Introduction

At1:

47AM（localtime）,Tuesday,September21,1999,adevastatingearthquakewithamagnitudeof7.3ontheRichterscalestruckcentralTaiwan.AccordingtotheseismicreportpublishedbytheTaiwanCentralWeatherBureau,theepicenterofthisearthquake,socalledChiChior921earthquake,islocatedat23.85Nand120.81Eatadepthof7.0km（Fig.1）.The921earthquakewasassociatedwithtwo

Figure1EpicenterandActiveFaultsinTaiwan

closelyspacedfaults,ChelungpuandShuangtungfaults（Faultlines18and20inFig.1）.Thesetwofaultsare10kmapartandalmostinparallel.ThehypocenteratthetownofChiChiisattheintersectionofthesetwofaults.ItwascausedbythereversivefaultmovementatthesubductionzoneboundaryofEuroasianandPhilippinoplates.Theofficialestimatesofthecasualtiesandlossesare2,161casualties,8736injuriesand$3.7billionpropertyloss（NSF/ROC1999）.ThisisthestrongestearthquaketohitTaiwanwithinthepast100yearsandthemostcostlynaturaldisaster.

Mostcasualtieswereduetonumerousfailuresofnon-ductileconcretebuildings.Sincetheearthquakestruckinthemiddleofthenight,veryfewcasualtieswerecausedbyfailuresofbridges.However,millionofdollarswerelostduetodamageorcollapseofbridges.Manydamagedbridgesalongthekeyrouteswererepairedonatemporarybasis.Otherswereputunderinvestigationtofindthestrategyofretrofitting.Basedontheseverityofthedamageandalsotheuseforstrategyofretrofitting,bridgefailuresduetothisearthquakeweredividedintothreegroups:

（1）Severecase:

Trafficwasinterruptedbyfailedbridgepiersorfallingbeams;

（2）Moderatecase:

Controlledtrafficwasimposedduetosettlement,damagedbearing,andcrackingofdecks,beamsorpiers;

（3）Minorcase:

Normaltrafficismaintainedwithslightsettlement,minorcracking,orminorhorizontalmovement.

HighwaydamagewaswidespreadthroughouttwocentralTaiwancounties,TaichungandNantou.Hundredsofbridgesfromexpresswaystocountyroadsarelocatedinthesetwocounties.Exceptforabout10%ofthebridgepopulationexperiencingmoderate-to-majordamage,mostescapedseriousdamage.Figure2

Figure2-WushiBridge,Epicenter,ChelungpuFaultlineandSurroundingStations

（Ref:

CentralWeatherBureau,TheMinistryofTransportationandCommunication,Taiwan,ROC）

showsmeasuringstationsandtheirmeasuredpeakgroundaccelerations（PGAs）.Onthetopofthecrossareverticalaccelerations,ontheleftareE-WhorizontalgroundaccelerationsandontherightareN-Shorizontalgroundaccelerations.Allthoseconsideredtobeunder'near-field'actionaresubjectedtointensehorizontalandverticalgroundmotionaswellassurfacefaultmovement.Theworstdisplacementcausedbythe921earthquakewasabout7-8metersalongcertainsectionsoftheChelungpufault.Itisunderstandablethatiffaultspassunderthebridgeandthedislocationsarelarge,catastrophicincidentsandevenbridgecollapsesareboundtohappen.

SiteInvestigation

Theseconddayaftertheearthquake,evenwiththeinterruptedtraffic,thefirstauthorledtwelvestudents,dividedintosixgroups,whorodemotorcyclestovisitthedamagedbridgesitesandtookhundredsofpicturestobuildalargeinventoryforfuturereference.Thesecondauthoralsovisitedthesitesamonthlatertocollectmoreinformation,assessthedamageandevaluatethecauses.WushiBridgeisoneofthe

bridgesinspectedandisreviewedinthispaper.Wushibridge,locatedacrosstheChelungpufaultline（Fig.2）,showsmultiplebridgefailuremodesandgivesarepresentativecaseofbridgefailureunderearthquake.Ingeneral,itmaybenoticedthatmostoftheproblemscanbeblamedondesignsbasedonearlycodesandtheseverityoftheearthmovement.Basedonthemeasurementrecordsalongthefaultline（Fig.2）,mostofthegroundaccelerationswereover300gal,someevenashighas1G,whicharemuchhigherthanthelatestdesigngroundaccelerationsof0.33G,0.28G,0.23Gand0.18G.Ifthemeasuredaccelerationrecordswereusedinthedesign,unexpectedseismicactionsandsoileffectscauseddamageandcollapses.

ThispapergivesabriefoverviewoftheinvestigationandpossiblecausesfordamagetoWushibridge.Thebridgeshowssomecomplexfailuremodescausedbythisearthquake.Anoverviewofthecausesofdamageandcollapseinthisearthquakemayofferreliableguidelinesondeficienciesinbridgedesignandlaterconsiderationsinimprovingcodeprovisions.

BridgeDamageInvestigation

WushiislocatedonProvisionRoute3andisanessentiallinkbetweenTaichungandNantoucounties.ThetotallengthoftheWushiBridgeis624.5metersandthetotalwidthis26meters.Thebridgeconsistsofonenorthboundbridgeandonesouthboundbridge,eachwithtwolanesof12.5meters（Fig.3）.Thenorthbound

Figure3-PlanandElevationofWushiBridge

bridgewasfinishedinthe1960'sandthesouthboundonewascompletedin1973.ThesuperstructuresusedprestressedconcreteI-beamswithconstantspanlengthof34.84meters.

Thecrosssectionofthenorthboundandsouthboundsuper-andsubstructuresareshowninFigure4.Itisnoticedthatthenorthboundsubstructuresarewalltype

Figure4-CrossSectionsofWushiBridge

concretepiersandthesouthboundsubstructuresarehammerheadconcretepiers.Theyareallsupportedby6-meterdiameter,13-16metershaftfoundations.

Mostofthedesignandconstructioninformationofthenorthboundbridge,constructedinthe1960's,isunavailable.ThesouthboundbridgeadoptedKh=0.15（equivalenttothepeakgroundaccelerationPGA=150gal）astheearthquakedesigncoefficient.Boringrecordsshowthattheriverbedwascoveredwithpebblesandunderlinedbyclayrock.

TheWushibridgeisarivercrossingbridgeanditincidentallycrossestheChelongpuFault.Thenorthernendofthebridgewasthemoreseverelydamaged.Onesideofthefaultwasliftedverticallyabout2.1-2.3meters.Movementalongthebridgeslongitudinaldirectionwasestimatedtobeabout2.2-2.3metersandthemovementalongthebridgestransversedirectionwasabout2.1-2.3meters.Thethirdspanofthenorthboundbridgecrossesthesubductionzoneboundarywith45reversefaultmovement.Themovementcausedthesuperstructureofthefirstseveralspanstofalltotheground.Inspection（Fig.5）showedclearevidenceoffaultupliftingonbothsidesofthefallenspans.

Figure5-ObliqueFaultMovementbetweenPiersP3NandP3S

AccordingtothepublishedrecordfromtheCentralWeatherBureau,theEast-WestPGAwas518gal,North-Southwas639gal,andverticalwas416gal.Thesurfacepermanenthorizontalmovementwas2.3metersand2.9metersalongtheEast-WestandNorth-South,respectively.

TherecordclosesttothesitewasmadeattheFreefieldStrongSeismicStationTCUD71,whichisabout5.5KmsoutheastofthenearesttownofChao-Fung.AftertheChi-Chiearthquake,Wushibridgewasseverelydamagedandtheroadwasclosed.Duetotheslipmovement,thefirstandsecondspansofthenorthboundbridgefelltothegroundandthethirdspanclearlyshowedlateralmovement（Fig.6）.Thenorthernendabutmentofthenorthboundbridgewaspushedbythespanandthejigsawtypeof

Figure6-FallenBeamduetoSuperstructureMovementintheLongitudinalDirectionatPierP2N

expansionjointandthebackwallboardweredemolished.Underthepressure,thebackfillwaspushedbythesuperstructureandmovedupward（Fig.7）。

Under

Figure7-BackfillUpwardMovementattheNorthAbutmentoftheNorthboundBridge

horizontalvibration,substantialshearcracksshowedonthesouthboundhammerheadconcretepier（Fig.8）.Thecapofthefirstpierofthenorthboundbridge（P1N）crackedandtheEastsideexteriorPCIbeamswereflexured.Theenddiaphragmsandshearblocksdesignedtopreventconcretebeamsfrommovinglaterallywerecrushedasthewholesuperstructuremovedwestward（Fig.9）.

Figure8-ShearCracksontheSouthboundHammerheadConcretePierp2S

Figure9-CrushedEndDiaphragmsandShearBlocksatPierP1N

ReviewofDesignCodes

Taiwanislocatedinanactiveseismicarea.BridgedesignbasedonearthquakesinTaiwanhasalonghistory.Therehavebeenthreemajorstages,startingfrom1960:

（1）November1960,theDepartmentofTransportationpublishedthefirsteditionof“HighwayBridgeEngineeringDesignSpecifications”（DOT/ROC1960）whichdividestheTaiwanareaintotwozoneswith0.1Gand0.15G,respectively.ThesecondeditionoftheSpecificationsmodifiedthecoverageofthehighseismiczoneintoonelargeareabutstillwithgroundaccelerationsof0.1Gand0.15G.In1970,thefirstfreewayconnectingnorthernandsouthernTaiwanwasintheplanningstage.Aspecialprojectfocusingonearthquakeswasalsounderwayandthecoefficients,basedonthegeographicarea,soilconditionandimportanceofthebridge,weremodifiedto0.2,0.15and0.1.

（2）January1987,theDepartmentofTransportationpublished“HighwayBridgeDesignSpecifications”（MTC/ROC1987）Basedonthelatestearthquaketheoryavailableatthattime,thedesignhorizontalcoefficientKhwasdeterminedby:

Kh=ZSIC0iftheheightof