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..State-of-the-artreportofbridgehealthmonitoringAbstractThedamagediagnosisandhealthmonitoringofbridgestructuresareactiveareasofresearchinrecentyears.Comparingwiththeaerospaceengineeringandmechanicalengineering,civilengineeringhasthespecialitiesofitsowninpractice.Forexample,becausebridges,aswellasmostcivilengineeringstructures,arelargeinsize,andhavequitelownaturalfrequenciesandvibrationlevels,atlowamplitudes,thedynamicresponsesofbridgestructurearesubstantiallyaffectedbythenonstructuralcomponents,unforeseenenvironmentalconditions,andchangesinthesecomponentscaneasilytobeconfusedwithstructuraldamage.Allthesegivethedamageassessmentofcomplexstructuressuchasbridgesastillchallengingtaskforbridgeengineers.Thispaperfirstlypresentsthedefinitionofstructuralhealthmonitoringsystemanditscomponents.Then,thefocusofthediscussionisplacedonthefollowingsections:①thelaboratoryandfieldtestingresearchonthedamageassessment;②analyticaldevelopmentsofdamagedetectionmethods,including(a)signatureanalysisandpatternrecognitionapproaches,(b)modelupdatingandsystemidentificationapproaches,(c)neuralnetworksapproaches;and③sensorsandtheiroptimumplacements.Thepredominanceandshortcomingsofeachmethodarecomparedandanalyzed.Recentexamplesofimplementationofstructuralhealthmonitoringanddamageidentificationaresummarizedinthispaper.Thekeyproblemofbridgehealthmonitoringisdamageautomaticdetectionanddiagnosis,anditisthemostdifficultproblem.Lastly,researchanddevelopmentneedsareaddressed.1IntroductionDuetoawidevarietyofunforeseenconditionsandcircumstance,itwillneverbepossibleorpracticaltodesignandbuildastructurethathasazeropercentprobabilityoffailure.Structuralaging,environmentalconditions,andreuseareexamplesofcircumstancesthatcouldaffectthereliabilityandthe..lifeofastructure.Thereareneedsofperiodicinspectionstodetectdeteriorationresultingfromnormaloperationandenvironmentalattackorinspectionsfollowingextremeevents,suchasstrong-motionearthquakesorhurricanes.Toquantifythesesystemperformancemeasuresrequiressomemeanstomonitorandevaluatetheintegrityofcivilstructureswhileinservice.SincetheAlohaBoeing737accidentthatoccurredonApril28,1988,suchinteresthasfosteredresearchintheareasofstructuralhealthmonitoringandnon-destructivedamagedetectioninrecentyears.AccordingtoHousner,etal.(1997),structuralhealthmonitoringisdefinedas“theuseofin-situ,non-destructivesensingandanalysisofstructuralcharacteristics,includingthestructuralresponse,fordetectingchangesthatmayindicatedamageordegradation”[1].Thisdefinitionalsoidentifiestheweakness.WhileresearchershaveattemptedtheintegrationofNDEwithhealthmonitoring,thefocushasbeenondatacollection,notevaluation.Whatisneededisanefficientmethodtocollectdatafromastructurein-serviceandprocessthedatatoevaluatekeyperformancemeasures,suchasserviceability,reliability,anddurability.So,thedefinitionbyHousner,etal.(1997)shouldbemodifiedandthestructuralhealthmonitoringmaybedefinedas“theuseofin-situ,nondestructivesensingandanalysisofstructuralcharacteristics,includingthestructuralresponse,forthepurposeofidentifyingifdamagehasoccurred,determiningthelocationofdamage,estimatingtheseverityofdamageandevaluatingtheconsequencesofdamageonthestructures”(Fig.1).Ingeneral,astructuralhealthmonitoringsystemhasthepotentialtoprovidebothdamagedetectionandconditionassessmentofastructure.Assessingthestructuralconditionwithoutremovingtheindividualstructuralcomponentsisknownasnondestructiveevaluation(NDE)ornondestructiveinspection.NDEtechniquesincludethoseinvolvingacoustics,dyepenetrating,eddycurrent,emissionspectroscopy,fiber-opticsensors,fiber-scope,hardnesstesting,isotope,leaktesting,optics,magneticparticles,magneticperturbation,X-ray,noisemeasurements,patternrecognition,pulse-echo,ra-diography,andvisualinspection,etc.Mostofthese..techniqueshavebeenusedsuccessfullytodetectlocationofcertainelements,cracksorwelddefects,corrosion/erosion,andsoon.TheFederalHighwayAdministration(FHWA,USA)wassponsoringalargeprogramofresearchanddevelopmentinnewtechnologiesforthenondestructiveevaluationofhighwaybridges.Oneofthetwomainobjectivesoftheprogramistodevelopnewtoolsandtechniquestosolvespecificproblems.Theotheristodeveloptechnologiesforthequantitativeassessmentoftheconditionofbridgesinsupportofbridgemanagementandtoinvestigatehowbesttoincorporatequantitativeconditioninformationintobridgemanagementsystems.Theyhopedtodeveloptechnologiestoquickly,efficiently,andquantitativelymeasureglobalbridgeparameters,suchasflexibilityandload-carryingcapacity.Obviously,acombinationofseveralNDEtechniquesmaybeusedtohelpassesstheconditionofthesystem.Theyareveryimportanttoobtainthedata-baseforthebridgeevaluation.ButitisbeyondthescopeofthisreviewreporttogetintodetailsoflocalNDE.Healthmonitoringtechniquesmaybeclassifiedasglobalandlocal.GlobalattemptstosimultaneouslyassesstheconditionofthewholestructurewhereaslocalmethodsfocusNDEtoolsonspecificstructuralcomponents.Clearly,twoapproachesarecomplementarytoeachother.Allsuchavailableinformationmaybecombinedandanalyzedbyexpertstoassessthedamageorsafetystateofthestructu