桥梁设计外文翻译.docx
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桥梁设计外文翻译.docx
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桥梁设计外文翻译
附录2外文资料翻译
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11.7.4Deflection
11.7.4.1DeadLoadandCreepDeflection
Globalverticaldeflectionsofsegmentalbox-girderbridgesduetotheeffectsofdeadloadandpost-tensioningaswellasthelong-termeffectofcreeparenormallypredictedduringthedesignprocessbytheuseofacomputeranalysisprogram.Thedeflectionsaredependent,toalargeextent,onthemethodofconstructionofthestructure,theageofthesegmentswhenpost-tensioned,andtheageofthestructurewhenotherloadsareapplied.Itcanbeexpected,therefore,thattheactualdeflectionsofthestructurewouldbedifferentfromthatpredictedduringdesignduetochangedassumptions.
Thedeflectionsareusuallyrecalculatedbythecontractor’sengineer,basedontheactualconstructionsequence.
11.7.4.2CamberRequirements
Thepermanentdeflectionofthestructureafterallcreepdeflectionshaveoccurred,normally10to15yearsafterconstruction,maybeobjectionablefromtheperspectiveofridingcomfortfortheusersorfortheconfidenceofthegeneralpublic.Evenifthereisnostructuralproblemwithaspanwithnoticeablesag,itwillnotinspirepublicconfidence.Forthesereasons,acamberwillnormallybecastintothestructuresothatthepermanentdeflectionofthebridgeisnearlyzero.Itmaybepreferabletoignorethecamber,ifitisotherwisenecessarytocastasaginthestructureduringonstruction.
11.7.4.3GlobalDeflectionDuetoLiveLoad
Mostdesigncodeshavealimitontheallowableglobaldeflectionofabridgespanduetotheeffectsofliveload.ThepurposeofthislimitistoavoidthenoticeablevibrationfortheuserandminimizetheeffectsofmovingloadiMPact.Whenstructuresareusedbypedestriansaswellasmotorists,thelimitsarefurthertightened.
11.7.4.4LocalDeflectionDuetoLiveLoad
Similartothelimitsofglobaldeflectionofbridgespans,therearealsolimitationsonthedeflectionofthelocalelementsofthebox-girdercrosssection.Forexample,theAASHTOSpecificationslimitthedeflectionofcantileverarmsduetoserviceliveloadplusiMPactto¹⁄₃ofthecantileverlength,exceptwherethereispedestrianuse[1].
11.7.5Post-TensioningLayout
11.7.5.1ExternalPost-Tensioning
Whilemostconcretebridgescastonfalseworkorprecastbeambridgeshaveutilizedpost-tensioninginductswhicharefullyencasedintheconcretesection,otherinnovationshavebeenmadeinprecastsegmentalconstruction.Especiallyprevalentinstructuresconstructedusingthespan-by-spanmethod,post-tensioninghasbeenplacedinsidethehollowcelloftheboxgirderbutnotencasedinconcretealongitslength.Thisisknowasexternalpost-tensioning.Externalpost-tensioningiseasilyinspectedatanytimeduringthelifeofthestructure,eliminatestheproblemsassociatedwithinternaltendons,andeliminatestheneedforusingexpensiveepoxyadhesivebetweenprecast
segments.Theproblemsassociatedwithinternaltendonsare
(1)misalignmentofthetendonsatsegmentjoints,whichcausesspalling;
(2)lackofsheathingatsegmentjoints;and(3)tendonpull-throughonspanswithtightcurvature(seeFigure11.39).ExternalprestressinghasbeenusedonmanyprojectsinEurope,theUnitedStates,andAsiaandhasperformedwell.
11.7.5
Theprovisionfortheadditionofpost-tensioninginthefutureinordertocorrectunacceptablecreepdeflectionsortostrengthenthestructureforadditionaldeadload,i.e.,futurewearingsurface,isnowrequiredbymanycodes.Ofthepositiveandnegativemomentpost-tensioning,10%isreasonable.Provisionsshouldbemadeforaccess,anchorageattachment,anddeviationoftheseadditionaltendons.External,unbondedtendonsareusedsothatungroutedductsintheconcretearenotleftopen.
11.8SeismicConsiderations
11.8.1DesignAspectsandDesignCodes
Duetotypicalvibrationcharacteristicsofbridges,itisgenerallyacceptedthatunderseismicloads,someportionofthestructurewillbeallowedtoyield,todissipateenergy,andtoincreasetheperiodofvibrationofthesystem.Thisyieldingisusuallyachievedbyeitherallowingthecolumnstoyieldplastically(monolithicdeck/superstructureconnection),orbyprovidingayieldingorasoftbearingsystem[6].
Thesameprinciplesalsoapplytosegmentalstructures,i.e.,thesegmentalsuperstructureneedstoresistthedemandsimposedbythesubstructure.Veryfewimplementationsofsegmentalstruc-turesarefoundinseismicallyactiveCalifornia,wheremostoftheresearchonearthquake-resistantbridgesisconductedintheUnitedStates.ThePineValleyCreekBridge,ParrotsFerryBridge,andNorwalk/ElSegundoLineOvercrossing,allofthembeinginCalifornia,areexamplesofsegmentalstructures;however,thesebridgesareallsegmentallycastinplace,withmildreinforcementcrossingthesegmentjoints.
SomeguidancefortheseismicdesignofsegmentalstructuresisprovidedinthelatesteditionoftheAASHTOGuideSpecificationsforDesignandConstructionofSegmentalConcreteBridges[2],whichnowcontainsachapterdedicatedtoseismicdesign.Theguideallowsprecast-segmentalconstructionwithoutreinforcementacrossthejoint,butspecifiesthefollowingadditionalrequire-
mentsforthesestructures:
•ForSeismicZonesCandD[1],eithercast-in-placeorepoxiedjointsarerequired.
•Atleast50%oftheprestressforceshouldbeprovidedbyinternaltendons.
•Theinternaltendonsaloneshouldbeabletocarry130%ofthedeadload.
Forotherseismicdesignanddetailingissues,thereaderisreferredtothedesignliteratureprovided
bytheCaliforniaDepartmentofTransportation,Caltrans,forcast-in-placestructures[5-8].
11.8.2Deck/SuperstructureConnection
Regardlessofthedesignapproachadopted(ductilitythroughplastichingingofthecolumnorthroughbearings),thedeck/superstructureconnectionisacriticalelementintheseismicresistantsystem.Abriefdescriptionofthedifferentpossibilitiesfollows.
11.8.2.1MonolithicDeck/SuperstructureConnection
Forthelongitudinaldirection,plastichingingwillformatthetopandbottomofthecolumns.Sincemostofthetestinghasbeenconductedoncast-in-placejoints,thiscontinuestobethepreferredoptionforthesecases.Forshortcolumnsandforsolidcolumns,thedetailinginthisareacanbereadilyadaptedfromstandardCaltranspracticeforcast-in-placestructures,asshownonFigure11.40.Thejointareaisthenessentiallydetailedsoitisnodifferentfromthatofafullycast-in-placebridge.Inparticular,aCaltransrequirementforpositivemomentreinforcementoverthepiercanbedetailedwithprestressingstrand,asshownbelow.Forlargespansandtallcolumns,hollowcolumnsectionswouldbemoreappropriate.Inthesecases,careshouldbetakentoconfinethemaincolumnbarswithcloselyspacedties,andjointshearreinforcementshouldbeprovidedaccordingtoReference[3or7].Theuseoffullyprecastpiersegmentsinsegmentalsuperstructureswouldprobablyrequirespecialapprovaloftheregulatinggovernmentagency,sincesuchasolutionhasnotyetbeentestedforbridgesandisnotcodified.Nevertheless,baseduponfirstprinciples,andwiththehelpofstrut–tiemodels,itispossibletodesignsystemsthatwouldworkinpractice[6].Thesegmentalsuperstructureshouldbedesignedtoresistatleast130%ofthecolumnnominalmomentusingthestrengthreductionfactorsprescribedinRef.[2].Offurtherinterestmaybeacombinationofprecastandcast-in-placejointasshowninFigure11.41,whichwasadaptedfromRef.[8].Here,theprecastsegmentservesasaformforthecast-in-placeportionthatfillsuptheremainderofthesolidpiercap.Otherideascanalsobederivedfromthebuildingindustrywheresomemodeltestinghasbeenperformed.Ofparticularinterestforbridgescouldbeasystemthatworksbyleavingdowelsinthecolumnsandsupplyingtheprecastsegmentwithmatchingformedholes,whicharegroutedafterthesegmentisslippedoverthereinforcement[9]
11.8.2.2Deck/SuperstructureConnectionviaBearings
Typically,forspansupto45merectedwiththespan-by-spanmethod,thesuperstructurewillbesupportedonbearings.Foractioninthelongitudinaldirection,elastomericorisolationbearingsarepreferredtoafixed-end/expansion-endarrangement,sincethesebetterdistributetheloadbetweenthebearings.Furthermore,thesebearingswillincreasetheperiodofthestructure,whichresultsinanoveralllowerinducedforcelevel(beneficialforhigher-frequencystructures),andisolationbearingswillprovidesomestructuraldampingaswell.Inthetransversedirection,thebearingsmaybeabletotransferloadbetweensuper-andsub-structurebysheardeformation;however,forthecaseswherethisisnotpossible,shearkeyscanbeprovidedasisshowninFigure11.42.Itshouldbenotedthatinregionsofhighseismicity,forstructureswithtallpiersorsoftsubstructures,thebearingdemandsmaybecomeexcessiveandamonolithicdeck–superstructureconnectionmaybecomenecessary.Forthestructure-on-bearingsapproach,theforcelevelforthesuperstructurecanbereadily,determined,sinceoncethebearingdemandsareobtainedfromtheanalysis,theycanbeappliedtothesuperstructureandsubstructure.Thesuperstructureshouldresisttheresultingforcesatultimate(usingtheapplicablecodeforce-reductionfactors),whereasthesubstructurecanbeallowedtoyieldplasticallyifnecessary.
11.8.2.3ExpansionHinges
Fromtheseismicpointofview,itisdesirabletoreducethenumberofexpansionhinges(EH)toaminimum.IfEHsareneeded,themostbeneficiallocationfromtheseismicpointofviewisatmidspan.ThiscanbeexplainedbyobservingFigure11.43,wherethesuperstructurebending
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