桥梁毕业设计外文翻译混凝土桥的结构处理工具Word格式文档下载.docx
《桥梁毕业设计外文翻译混凝土桥的结构处理工具Word格式文档下载.docx》由会员分享,可在线阅读,更多相关《桥梁毕业设计外文翻译混凝土桥的结构处理工具Word格式文档下载.docx(7页珍藏版)》请在冰豆网上搜索。
Reliabilityanalysis:
astructuresmanagementtoolforconcretebridges
Reinforcedconcretestructuresaresusceptibletoavarietyofdeteriorationmechanisms,includingalkali-thawactionandchlorideingress.Substantialresearchhasbeenundertakeninrelationtothesemechanismsandotherproblems.Thishasparticularlybeenthecaseoverthelast20yearsorso,wheretheobjectivehasbeentoidentifycauses,consequencesanddevelopremediationstrategies.Thishasimprovedunderstandingoflong-termbehaviourofreinforcedconcreteandresultedinthedevelopmentoftechniquestoincreasedeteriorationresistance.
Atpresent,themostcommonapproachistoactafteraproblemhasbeenidentified,knownasre-activemaintenance.Thismaynotbethemosteconomicsolutionsince,inmanycases,maintenanceismorecostlythanpreventativetreatments.However,ownersareoftenreluctanttopayforpreventativetreatmentsbeforedeteriorationisapparent.Earlyapplicationoftreatmentsmaynotbetheoptimalsolutioninthelongrun.Integrateddeteriorationandperformancepredictionmodelingisessentialtopro-activelyplanandprioritiseinspection,testingandmaintenance.Thisbecomesincreasinglyimportantasinfrastructureagesandjustificationformaintenancefundingbecomesincreasinglycritical.
Performanceassessmentcanbeachievedthroughsurveys,testingandformalcalculations,ideallybasedonsitedatathatrepresent,asaccuratelyaspossible,thestateofthestructure.Byintegratingpredictivedeteriorationmodelswithassessmenttoolsandperformancecriteria(atelement,structureorgrouplevel)itbecomespossibletobasethemaintenanceregimeontime-dependentperformanceprofiles.Thisisparticularlyrelevantinthecontextofwhole-wifecostingprocedures.
Substantialresearchhasbeenundertakeninrelationtothesemechanismsandotherproblems.Thishasparticularlybeenthecaseoverthelast20yearsorso,wheretheobjectivehasbeentoidentifycauses,consequencesanddevelopremediationstrategies.Thishasimprovedunderstandingoflong-termbehaviourofreinforcedconcreteandresultedinthedevelopmentoftechniquestoincreasedeteriorationresistance.
Atpresent,themostcommonapproachistoactafteraproblemhasbeenidentified,knownasre-activemaintenance.Thismaynotbethemosteconomicsolutionsince,inmanycases,maintenanceismorecostlythanpreventativetreatments.However,ownersareoftenreluctanttopayforpreventativetreatmentsbeforedeteriorationisapparent.Earlyapplicationoftreatmentsmaynotbetheoptimalsolutioninthelongrun.Integrateddeteriorationandperformancepredictionmodelingisessentialtopro-activelyplanandprioritiseinspection,testingandmaintenance.Thisbecomesincreasinglyimportantasinfrastructureagesandjustificationformaintenancefundingbecomesincreasinglycritical.
Reliabilityanalysishasemergedasanimportanttoolinthismulti-objectivemanagementprocess,whichmusttakeintoaccountsafety,functionalityandsustainabilitycriteria.Insimpleterms,thereliabilityofastructureorasystemistheprobabilityofachievingaparticularperformancelevel.Probabilityorlikelihoodistheappropriatemeasure,sinceallengineeringsystemsaresusceptibletouncertainties,arisingfromrandomphenomenaandincompleteknowledge.Reliabilityanalysisinstructuralengineeringenablesquantificationofuncertaintiesassociatedwithloading,materials,deterioration,modelingandotherfactors.Theseareintegratedintoamethodthatestimatestheprobabilityofreachingthespecifiedperformancelevelduringtheservicelifeofastructure.Themethodisincreasinglybeingusedinbridgeengineering,bothforcalibrationofsafetylevelsincodesandstandardsandimprovingandrefiningassessmentmethodologies.Thepurposeofthisarticleistooutlineitsapplicationinmanagingbridgessusceptibletodeterioration.
Althoughdataformanydeteriorationvariablescanbederivedfromlaboratorystudies,thereisanabsenceofsimilardatarealstructures.Animportantfeatureofthemodelisthefacilitytomodifyinitialpredictions,basedonpublished(knownas‘prior’)data,usinginformationanddataobtaineddirectlyfromtheactualstructures.Reliabilityanalysisisappropriateforthispursoseasifitcanreadilyincorporateadditionaldata,updatingtheprobabilityofreachingaperformancetarget.Theconceptisanalogoustoupdatingtheprobabilityofarrivingontimewhilstonatrain,havingjustobtainsomeextrainformationregardingtheoperatingconditionsahead.
Typicalresultsproducedbytheprobabilisticdeteriorationmodelforacrossbeamchlorideexposurezone,similartothedelaminatedareashowninFigure3,areshowninFigure4.Assumingathresholdof40%initiationisspecifiedforthefirstinspection,themodelsuggeststhatitshouldbeundertakenaftereightyears.Assumingthatinspectionindicatessignificantlylesscorrosioninitiation(e.g.onlyabout10%)andattributed,throughsiteinvestigations,toconcretecoverbeinghigherthanexpected,arevisedpredictionoftheperformanceprofilemaybegenerated.Thebridgemanagementactionsmaythenbealteredaccordingly.
Figure5illustrateshowalimitstateprofileforthiszonechangeswithassumedconditions.Thedeteriorationmodelhasbeenintegratedwithbondlimitstateequations.Thus,assumingthatthecomponenthasatargetnominalprobabilityoffailureof1×
10-5peryear,profile1suggestsalifetimeofonly17to18years,whereasProfile2suggests30to31years.Bothareshortlifetimeswhencomparedwiththenormallifeexpectancyofbridges.However,initialconditionsrelatingtotheseresultsassumethatthedeckjointhasfailedfromtheoutset.Alternatively,theexpressionsformodellingbondstrengthmaybeover-conservative,astheyweredevelopedforintactstructures.
Bridgeperformancecriteria
CurrentUKassessmentcodesareconcernedwithultimatelimitstates(ULS)anddonotexplicitlyrequirecheckingofserviceabilitylimitstates(ULS).ItisassumedthatanexistingstructurehasexperiencedSLSloadsduringitslife.However,thewidelyacceptedSLScriteriaofdeflectionandcrackingdonotfullytakeintoaccounttheproblemsposedbydeterioration.Deterioration-basedcriteriasuchasruststaining,delaminationandspallingneedtobeconsideredbecausetheyclearlyinfluencebridgeperformance,bothfunctionalandfinancial.Theseoftenprovethedominantfactorwithregardtobridgemanagementstrategy.
Byexplicitlyconsideringandspecifyingperformancelevels,theengineerisawareoftheimportantdeteriorationindicatorsinordertoestablishtheinspectionandmaintenanceregimefortheparticularstructure/member.Theseperformancelevelsmaychangeovertime,duetochangesinfunction,loading,structureimportanceetc.forexample,therelationshipbetweenactualandrequiredperformanceisconceptualizedbythediagramshowninFigure1.Thus,reliabilityanalysismaybeusedtoformulatetheprobabilitythatperformancewillexceedthatrequired,therebyestimatingthereliabilityofthestructure.Theperformancemeasurecanberelatedtosafety,functionalityoranyotherappropriatecriterion.
Modelingchloride-induceddeterioration
Thisparticularprojectconcentratedononespecificareaofreinforcedconcretedeterioration,specificallyarisingfromchlorideingress.Chloridesarepresentinde-icingsaltsusedintheUKduringwinter.Chlorideionsmigratethoughtheconcrete,e.g.byabsorptionanddiffusion.Undersuitableconditions,theyinitiatereinforcementbarcorrosion.Thecorrosionmechanismproducesrust.Theincreasedvolumeofthemetal,duetotherust,leadstocracking,delaminationandspallingoftheconcretecover.Thisresultsinmorerapidandextensivereinforcementcorrosion.
Reinforcedconcretebridgeelementslocatedbelowexpansionjointsareparticularlysusceptibletochlorideattackifthejointfails.HighwayviaductsintheUKtypicallyconsistofareinforcedconcretecrossbeamdirectlylocatedbelowtheexpansionjoint,(seeFigure2).Manycrossbeamshavesufferedseverereinforcementcorrosion,delaminationandspalling.AtypicalexampleisshowninFigure3,wherethereinforcementcoveroverthecrossbeamhasdelaminated.
Aprobabilisticdeteriorationmodelforreinforcedconcretebridgecomponentswasdeveloped,takingintoaccountthecharacteristicsofthesestructuresandtheirenvironment.Itassumesthatbothdiffusionandabsorptionplayapartinchlotidemigrationthroughtheconcrete,thevariabilityinthequantityofde