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原文1AUTOMATICDEFLECTIONANDTEMPERATURE

MONITORINGOFABALANCEDCANTILEVERCONCRETEBRIDGEbyOlivierBURDET,Ph.D.SwissFederalInstituteofTechnology,Lausanne,Switzerland

InstituteofReinforcedandPrestressedConcreteSUMMARYThereisaneedforreliablemonitoringsystemstofollowtheevolutionofthebehaviorofstructuresovertime.Deflectionsandrotationsarevaluesthatreflecttheoverallstructurebehavior.Thispaperpresentsaninnovativeapproachtothemeasurementoflong-termdeformationsofbridgesbyuseofinclinometers.Highprecisionelectronicinclinometerscanbeusedtofolloweffectivelylong-termrotationswithoutdisruptionofthetraffic.Inadditiontotheiraccuracy,theseinstrumentshaveproventobesufficientlystableovertimeandreliableforfieldconditions.TheMentuebridgesaretwin565mlongbox-girderpost-tensionedconcretehighwaybridgesunderconstructioninSwitzerland.Thebridgesarebuiltbythebalancedcantilevermethodoveradeepvalley.Thepiersare100mhighandthemainspanis150m.Acentralizeddataacquisitionsystemwasinstalledinonebridgeduringitsconstructionin1997.Everyminute,thesystemrecordstherotationandtemperatureatanumberofmeasuringpoints.Thesimultaneousmeasurementofrotationsandconcretetemperatureatseverallocationsgivesaclearideaofthemovementsinducedbythermalconditions.Thesystemwillbeusedincombinationwithahydrostaticlevelingsetuptofollowthelong-termbehaviorofthebridge.Preliminaryresultsshowthatthesystemperformsreliablyandthattheaccuracyofthesensorsisexcellent.Comparisonoftheevolutionofrotationsandtemperatureindicatethatthestructurerespondstochangesinairtemperatureratherquickly.BACKGROUNDAllovertheworld,thenumberofstructuresinservicekeepsincreasing.Withthedevelopmentoftrafficandtheincreaseddependenceonreliabletransportation,itisbecomingmoreandmorenecessarytoforeseeandanticipatethedeteriorationofstructures.Inparticular,forstructuresthatarepartofmajortransportationsystems,rehabilitationworksneedtobecarefullyplannedinordertominimizedisruptionsoftraffic.Automaticmonitoringofstructuresisthusrapidlydeveloping.Long-termmonitoringofbridgesisanimportantpartofthisoverallefforttoattempttominimizeboththeimpactandthecostofmaintenanceandrehabilitationworkofmajorstructures.Byknowingtherateofdeteriorationofagivenstructure,theengineerisabletoanticipateandadequatelydefinethetimingofrequiredinterventions.Conversely,interventionscanbedelayeduntiltheconditionofthestructurerequiresthem,withoutreducingtheoverallsafetyofthestructure.Thepaperpresentsaninnovativeapproachtothemeasurementoflong-termbridgedeformations.Theuseofhighprecisioninclinometerspermitsaneffective,accurateandunobtrusivefollowingofthelong-termrotations.Themeasurementscanbeperformedundertrafficconditions.Simultaneousmeasurementofthetemperatureatseverallocationsgivesaclearideaofthemovementsinducedbythermalconditionsandthoseinducedbycreepandshrinkage.ThesystempresentedisoperationalsinceAugust1997intheMentuebridge,currentlyunderconstructioninSwitzerland.Thestructurehasamainspanof150mandpiers100mhigh.LONG-TERMMONITORINGOFBRIDGESAspartofitsresearchandserviceactivitieswithintheSwissFederalInstituteofTechnologyinLausanne(EPFL),IBAP-ReinforcedandPrestressedConcretehasbeeninvolvedinthemonitoringoflong-timedeformationsofbridgesandotherstructuresforovertwenty-fiveyears[1,2,3,4].Inthepast,IBAPhasdevelopedasystemforthemeasurementoflong-termdeformationsusinghydrostaticleveling[5,6].ThissystemhasbeeninsuccessfulserviceintenbridgesinSwitzerlandforapproximatelytenyears[5,7].Thesystemisrobust,reliableandsufficientlyaccurate,butitrequireshumaninterventionforeachmeasurement,andisnotwellsuitedforautomaticdataacquisition.Oneadditionaldisadvantageofthissystemisthatitisonlyeasilyapplicabletoboxgirderbridgeswithanaccessiblebox.Occasionalcontinuousmeasurementsoverperiodsof24hourshaveshownthattheamplitudeofdailymovementsissignificant,usuallyamountingtoseveralmillimetersoveracoupleofhours.Thisisexemplifiedinfigure1,wheremeasurementsofthetwinLutrivebridges,takenoveraperiodofseveralyearsbeforeandaftertheywerestrengthenedbypost-tensioning,areshownalongwithmeasurementsperformedoveraperiodof24hours.Thescatterobservedinthedataisprimarilycausedbythermaleffectsonthebridges.Inthecaseofthesebox-girderbridgesbuiltbythebalancedcantilevermethod,withamainspanof143.5m,theamplitudeofdeformationsonasunnydayisofthesameorderofmagnitudethanthelongtermdeformationoverseveralyears.Instantaneousmeasurements,asthosemadebyhydrostaticleveling,arenotnecessarilyrepresentativeofthemeanpositionofthebridge.Thisoccursbecausethepositionofthebridgeatthetimeofthemeasurementisinfluencedbythetemperaturehistoryoverthepastseveralhoursanddays.Evenifeverycarewastakentoperformthemeasurementsearlyinthemorningandatthesameperiodeveryyear,ittookarelativelylongtimebeforeitwasrealizedthattheretrofitperformedontheLutrivebridgesin1988byadditionalpost-tensioning[3,7,11]hadnothadthesameeffectonbothofthem.

Figure1:Long-termdeflectionsoftheLutrivebridges,comparedtodeflectionsmeasuredina24-hourperiodAutomaticdataacquisition,allowingfrequentmeasurementstobeperformedatanacceptablecost,isthushighlydesirable.Astudyofpossiblesolutionsincludinglaser-basedleveling,fiberopticssensorsandGPS-positioningwasperformed,withtheconclusionthat,providedthattheirlong-termstabilitycanbedemonstrated,currenttypesofelectronicinclinometersaresuitableforautomaticmeasurementsofrotationsinexistingbridges[8].MENTUEBRIDGESTheMentuebridgesaretwinbox-girderbridgesthatwillcarrythefutureA1motorwayfromLausannetoBern.Eachbridge,similarindesign,hasanoveralllengthofapproximately565m,andawidthof13.46m,designedtocarrytwolanesoftrafficandanemergencylane.Thebridgescrossadeepvalleywithsteepsides(fig.2).Thebalancedcantileverdesignresultsfromabridgecompetition.The100mhighconcretepierswerebuiltusingclimbingformwork,afterwhichtheconstructionofthebalancedcantileverstarted(fig.3).&6501LausanneFiiiire3：Pier3of&6501LausanneFiiiire3：Pier3oftheMentuebridgediniEiaThefireecnnrileveridieINCLINOMETERSStartingin1995,IBAPinitiatedaresearchprojectwiththegoalofinvestigatingthefeasibilityofameasurementsystemusinginclinometers.Preliminaryresultsindicatedthatinclinometersofferseveraladvantagesfortheautomaticmonitoringofstructures.Table1summarizesthemainpropertiesoftheinclinometersselectedforthisstudy.Oneinterestingpropertyofmeasuringastructure’srotations,isthat,foragivenratioofmaximumdeflectiontospanlength,themaximumrotationisessentiallyindependentfromitsstaticsystem[8].Sincemaximalallowablevaluesofabout1/1,000forlong-termdeflectionsunderpermanentloadsaregenerallyacceptedvaluesworldwide,developmentsmadeforbox-girderbridgeswithlongspans,asisthecaseforthisresearch,areapplicabletootherbridges,

forinstancebridgeswithshorterspansandothertypesofcross-sections.Thisissignificantbecauseoftheneedtomonitorsmallerspanswhichconstitutethemajorityofallbridges.Table1:TvfainpropertiesoFinclinometersPropertyDescriptionAbpointeuieasuremeutHiemeasLireis,absolute,,relativeonlytotheinitialposition-Incaseoffailureofaninstnwient..theiiiformatioiigiven,bytheothersisstillusable.EasycoimecfiojitodataLucImcnnctcTSEireclectromcinstnunciitsthatproduceelectricsignalseasilycapturedbystandarddata兜匚quisitiousystems.Theinchocniictcr&usedforthisprojectinclude?snind.u^trialnetworkinterface(RS485)whichgjeatlyleducescabEstig-Principleofapplicablet&altTheamplitLideofrotationsisefisenliallyindependentofthestaticsystemorthecrosssrcticuofabridge.Becauseoftheirreliability,inclinometerscanVelocateditiliaidtoleachplaces7andaiethussiiiiableforallkindsctfcro^ssections.HighTlielughaccuracyofiii521iaom.etersopea.5possibilitiesformeasiureme-ntsofversrsmallmovciLLeiitS-.suchasthosethatoccurwithiiiacoupleminutes.ndoperaiionliiclmoiLiietersareverycompactingtrnjnientsthatrequireonlyandninialspace.Theselectedmodelincludesautomatictenrperatiirecompens-ation.Thesenderscanbeeasilyreplacedandarereusable.CoitAlthoughrelativelyhigh,thec-ostofinclinotiieterscompetitivewithcctiiparablesystems,andthecostofiirstallalicnaisratherlour?TheselectedinclinometersareoftypeWylerZerotronic+1°[9].Theiraccuracyis1microradian(口rad),whichcorrespondstoarotationofonemillimeterperkilometer,averysmallvalue.Foranintermediatespanofacontinuousbeamwithaconstantdepth,amid-spandeflectionof1/20,000wouldinduceamaximumrotationofabout150口rad,or0.15milliradians(mrad).Onepotentialproblemwithelectronicinstrumentsisthattheirmeasurementsmaydriftovertime.Toquantifyandcontrolthisproblem,amechanicaldevicewasdesignedallowingtheinclinometerstobepreciselyrotatedof180°inanhorizontalplane(fig.4).Thedriftofeachinclinometercanbeverysimplyobtainedbycomparingthevaluesobtainedintheinitialandrotatedpositionwithpreviouslyobtainedvalues.Sofar,ithasbeenobservedthatthetypeofinclinometerusedinthisprojectisnotverysensitivetodrifting.150mmAxisofrotaticniXRototingimver工45:士上壬5:=r:-r.-r.5!iii-i=r:-r.5:siConsoletothebridge150mmAxisofrotaticniXRototingimver工45:士上壬5:=r:-r.-r.5!iii-i=r:-r.5:siConsoletothebridgeFisiwc4:RotationsystemfbrthecanxctianofdriftingovertimeINSTRUMENTATIONOFTHEMENTUEBRIDGESBecauseanumberofbridgesbuiltbythebalancedcantilevermethodhaveshownanunsatisfactorybehaviorinservice[2,7,10],itwasdecidedtocarefullymonitortheevolutionofthedeformationsoftheMentuebridges.Thesebridgesweredesignedtakingintoconsiderationrecentrecommendationsforthechoiceoftheamountofposttensioning[7,10,13].Monitoringstartingduringtheconstructionin1997andwillbepursuedafterthebridgesareopenedtotrafficin2001.Deflectionmonitoringincludestopographiclevelingbythehighwayauthorities,anhydrostaticlevelingsystemovertheentirelengthofbothbridgesandanetworkofinclinometersinthemainspanoftheNorthbridge.Datacollectioniscoordinatedbytheengineerofrecord,tofacilitatecomparisonofmeasuredvalues.Theinformationgainedfromtheseobservationswillbeusedtofurtherenhancethedesigncriteriaforthattypeofbridge,especiallywithregardtotheamountofpost-tensioning[7,10,11,12,13].Theautomaticmonitoringsystemisdrivenbyadataacquisitionprogramthatgathersandstoresthedata.Thissystemisabletocontrolvarioustypesofsensorssimultaneously,atthepresenttimeinclinometersandthermalsensors.Thecomputerprogramdrivingalltheinstrumentationoffersaflexibleframework,allowingthelateradditionofnewsensorsordataacquisitionsystems.TheuseofthedevelopmentenvironmentLabView[14]allowedtoleveragethelargeuserbaseinthefieldoflaboratoryinstrumentationanddataanalysis.Thedataacquisitionsystemrunsonarathermodestcomputer,withanIntel486/66Mhzprocessor,16MBofmemoryanda500MBharddisk,runningWindowsNT.Allsensordataaregatheredonceperminuteandstoredincompressedformontheharddisk.Thesystemislocatedinthebox-girderontopofpier3(fig.5).Itcanwithstandsevereweatherconditionsandwillrestartitselfautomaticallyafterapoweroutage,whichhappenedfrequentlyduringconstruction.SENSORSFigure5(a)showsthelocationoftheinclinometersinthemainspanoftheNorthbridge.Thesensorsareplacedattheaxisofthesupports(①and⑤)，at1/4and3/4(③and④)ofthespanandat1/8ofthespanfor②.Inthecrosssection,thesensorsarelocatedontheNorthweb,ataheightcorrespondingtothecenterofgravityofthesection(fig.5a).ThesensorsareallconnectedbyasingleRS-485cabletothecentraldataacquisitionsystemlocatedinthevicinityofinclinometer①.Monitoringofthebridgestartedalreadyduringitsconstruction.Inclinometers①，②and③wereinstalledbeforethespanwascompleted.Theresultingmeasurementweredifficulttointerpret,however,becauseofthewidevariationsofanglesinducedbythevariousstagesofthisparticularmethodofconstruction.daiaacquisitjonsystemP3 P2(a)locaticmoftheinclinametersinrhemain5pmoftheNorthbridge(b)principlefortfcedtterminarififioftlifdeflecfedshape-from rotationsFigure5;LocationoftheuicluiQnicteT?!.jilth宕main即anandpiuiQipk￡01thercGomtnjctioHofThedeflectedshapewillbedeterminedbyintegratingthemeasuredrotationsalongthelengthofthebridge(fig.5b).Althoughthisintegrationisinprinciplestraightforward,ithasbeenshown[8,16]thatthetypeofloadingandpossiblemeasurementerrorsneedtobecarefullytakenintoaccount.North SouthFigure6:Locationo-fthennalsensoisiniliemainspanThermalsensorswereembeddedinconcretesothattemperatureeffectscouldbetakenintoaccountfortheadjustmentofthegeometryoftheformworkforsubsequentcasts.Figure6showsthelayoutofthermalsensorsinthemainspan.Themeasurementsectionsarelocatedatthesamesectionsthantheinclinometers(fig.5).Allsensorswereplacedintheformworkbeforeconcretingandwereoperationalassoonastheformworkwasremoved,whichwasrequiredfortheneedsoftheconstruction.Ineachsection,sevenoftheninethermalsensor(indicatedinsolidblackinfig.6)arenowautomaticallymeasuredbythecentraldataacquisitionsystem.RESULTSFigure7showstheresultsofinclinometrymeasurementsperformedfromtheendofSeptembertothethirdweekofNovember1997.Allinclinometersperformedwellduringthatperiod.Occasionalinterruptionsofmeasurement,asobservedforexampleinearlyOctoberareduetointerruptionofpowertothesystemduringconstructionoperations.Theoverallsymmetryofresultsfrominclinometers：-E「、''二=seemtoindicatethattheinstrumentsdriftisnotsignificantforthattimeperiod.Themaximumamplitudeofbridgedeflectionduringtheobservedperiod,estimatedonthebasisoftheinclinometersresults,isaround40mm.Moreaccuratevalueswillbecomputedwhenthemethodofdeterminationofdeflectionswillhavebeenfurthercalibratedwithothermeasurements.Severalperiodsofincrease,respectivelydecrease,ofdeflectionsoverseveraldayscanbeobservedinthegraph.Thisfurtherillustratestheneedforcontinuousdeformationmonitoringtoaccountforsucheffects.Themeasurementperiodwas.busy.intermsofconstruction,andincludedthefollowingoperations:thefinalconcretepoursinthatspan,horizontaljackingofthebridgetocompensatesomepiereccentricities,aswellasthestressingofthecontinuitypost-tensioning,andthede-tensioningoftheguycables(fig.3).Asaconsequence,theinterpretationofthesemeasurementsisquitedifficult.ItisexpectedthatAnglevariation[mrad]Figuiy7： snslevaiiafions TheendofSeptember〔hruughNflveinber1997furthermeasurements,madeafterthecompletionofthebridge,willbesimplertointerpret.Figure8showsadetailofthemeasurementsmadeinNovember,whilefigure.9showstemperaturemeasurementsatthetopandbottomofthesectionatmid-spanmadeduringthatsameperiod.Itisclearthatthemeasureddeflectionscorrespondtochangesinthetemperature.Thetemperatureatthebottomofthesectionfollowscloselyvariationsoftheairtemperature(measuredintheshadenearthenorthwebofthegirder).Ontheotherhand,thetemperatureatthetopofthecrosssectionislesssubjecttorapidvariations.Thismaybeduetothehighelevationofthebridgeaboveground,andalsotothefactthat,duringthemeasuringperiod,therewaslittledirectsunshineonthedeck.Thetemperaturegradientbetweentopandbottomofthecrosssectionhasadirectrelationshipwithshort-termvariations.Itdoesnot,however,appeartoberelatedtothegeneraltendencytodecreaseinrotationsobservedinfig.8.

Anglevariation[mrad]DateFigure9:TemperaturesinNoveinbei,1997FUTUREDEVELOPMENTSFuturedevelopmentswillincludealgorithmstoreconstructdeflectionsfrommeasuredrotations.Toenhancetheaccuracyofthereconstructionofdeflections,a3Dfiniteelementmodeloftheentirestructureisinpreparation[15].Thismodelwillbeusedtoidentifytheinfluenceonrotationsofvariousphenomena,suchascreepofthepiersandgirder,differentialsettlements,horizontalandverticaltemperaturegradientsortrafficloads.MuchworkwillbedevotedtotheinterpretationofthedatagatheredintheMentuebridge.Thefinalpartoftheresearchprojectworkwillfocusontwoaspects:understandingtheverycomplexbehaviorofthestructure,anddeterminingthemostimportantparameters,toallowasimpleandeffectivemonitoringofthebridgesdeflections.Finally,theresearchreportwillproposeguidelinesfordeterminationofdeflectionsfrommeasuredrotationsandpracticalrecommendationsfortheimplementationofmeasurementsystemsusinginclinometers.Itisexpectedthatwithinthecomingyearnewsiteswillbeequippedwithinclinometers.Experiencesmadebyusinginclinometerstomeasuredeflectionsduringloadingtests[16,17]haveshownthatthemethodisveryflexibleandcompetitivewithotherhigh-techmethods.Asanextensiontothecurrentresearchproject,aninnovativesystemforthemeasurementofbridgejointmovementisbeingdeveloped.Thissystemintegrateseasilywiththeexistingmonitoringsystem,becauseitalsousesinclinometers,althoughfromaslightlydifferenttype.CONCLUSIONSAninnovativemeasurementsystemfordeformationsofstructuresusinghighprecisioninclinometershasbeendeveloped.Thissystemcombinesahighaccuracywitharelativelysimpleimplementation.Preliminaryresultsareveryencouragingandindicatethattheuseofinclinometerstomonitorbridgedeformationsisafeasibleandoffersadvantages.Thesystemisreliable,doesnotobstructconstructionworkortrafficandisveryeasilyinstalled.Simultaneoustemperaturemeasurementshaveconfirmedtheimportanceoftemperaturevariationsonthebehaviorofstructuralconcretebridges.REFERENCESANDREYD.,Maintenancedesouvragesd'art:methodologiedesurveillance,PhDDissertationNr679,EPFL,Lausanne,Switzerland,1987.⑵BURDETO.,LoadTestingandMonitoringofSwissBridges,CEBInformationBulletinNr219,SafetyandPerformanceConcepts,Lausanne,Switzerland,1993.BURDETO.,Criterespourlechoixdelaquantitedeprecontraintedecoulantde{.observationdepontsexistants,CUST-COS96,Clermont-Ferrand,France,1996.HASSANM.,BURDETO.,FAVRER.,CombinationofUltrasonicMeasurementsandLoadTestsinBridgeEvaluation,5thInternationalConferenceonStructuralFaultsandRepair,Edinburgh,Scotland,UK,1993.FAVRER.,CHARIFH.,MARKEYI.,Observationalongtermedeladeformationdesponts,MandatdeRecherchede1’OFR86/88,FinalReport,EPFL,Lausanne,Switzerland,1990.FAVRER.,MARKEYI.,Long-termMonitoringofBridgeDeformation,NATOResearchWorkshop,BridgeEvaluation,RepairandRehabilitation,NATOASIseriesE:vol.187,pp.85-100,Baltimore,USA,1990.FAVRER.,BURDETO.etal.,Enseignementstiresd'essaisdechargeetd'observationsalongtermepour1'evaluationdespontsetlechoixdelaprecontrainte,OFRReport,83/90,Zurich,Switzerland,1995.DAVERIOR.,Mesuresdesdeformationsdespontsparunsystemed'inclinometrie,RapportdemaitriseEPFL-IBAP,Lausanne,Switzerland,1995.WYLERAG.,TechnicalspecificationsforZerotronicInclinometers,Winterthur,Switzerland,1996.FAVRER.,MARKEYI.,GeneralizationoftheLoadBalancingMethod,12thFIPCongress,PrestressedConcreteinSwitzerland,pp.32-37,Washington,USA,1994.FAVRER.,BURDETO.,CHARIFH.,Criterespourlechoixd'uneprecontrainte:applicationaucasd'unrenforcement,"ColloqueInternationalGestiondesOuvragesd’Art:QuelleStrategiepourMainteniretAdapterlePatrimoine,pp.197-208,Paris,France,1994.FAVRER.,BURDETO.,WahleinergeeignetenVorspannung,Beton-undStahlbetonbau,Beton-undStahlbetonbau,92/3,67,Germany,1997.FAVRER.,BURDETO.,Choixd'unequantiteapproprieedeprecontrainte,SIAD0129,Zurich,Switzerland,1996.NATIONALINSTRUMENTS,LabViewUser.sManual,Austin,USA,1996.BOUBERGUIGA.,ROSSIERS.,FAVRER.etal,Calculnonlineairedubetonarmeetprecontraint,RevueFrangaisduGenieCivil,vol.1n°3,Hermes,Paris,France,1997.FESTE.,Systemedemesureparinclinometrie:developpementd'unalgorithmedecalculdesfleches,MemoiredemaitrisedeDEA,Lausanne/Paris,Switzerland/France,1997.PERREGAUXN.etal.,VerticalDisplacementofBridgesusingtheSOFOSystem:aFiberOpticMonitoringMethodforStructures,12thASCEEngineeringMechanicsConference,SanDiego,USA,tobepublished,1998.原文2TheStructureofConcreteBridgePre-stressedconcretehasprovedtobetechnicallyadvantageous,economicallycompetitive,andestheticallysuperiorbridges,fromveryshortspanstructuresusingstandardcomponentstocable-stayedgirdersandcontinuousboxgirderswithclearspansofnearly100aft.Nearlyallconcretebridges,eventhoseofrelativelyshortspan,arenowpre-stressed.Pre-casting,cast-in-placeconstruction,oracombinationofthetwomethodsmaybeused.Bothpre-tensioningandposttensioningareemployed,oftenonthesameproject.IntheUnitedStates,highwaybridgesgenerallymust-meetloading,design,andconstructionrequirementsoftheAASHTOSpecification.Designrequirementsforpedestriancrossingsandbridgesservingotherpurposesmaybeestablishedbylocalorregionalcodesandspecifications.ACICodeprovisionsareoftenincorporatedbyreference.Bridgesspanstoabout100ftoftenconsistofpre-castintegral-deckunits,whichofferlowinitialcost,minimum,maintenance,andfasteasyconstruction,withminimumtrafficinterruption.Suchgirdersaregenerallypre-tensioned.Theunitsareplacedsidebyside,andareoftenpost-tensionedlaterallyatintermediatediaphragmlocations,afterwhichshearkeysbetweenadjacentunitsarefilledwithnon-shrinkingmortar.Forhighwayspans,anasphaltwearingsurfacemaybeapplieddirectlytothetopofthepre-castconcrete.Insomecases,acast-in-placeslabisplacedtoprovidecompositeaction.Thevoidedslabsarecommonlyavailableindepthsfrom15to21in.andwidthsof3to4ft.ForastandardhighwayHS20loading,theyaresuitableforspanstoabout50ft,Standardchannelsectionsareavailableindepthsfrom21to35inavarietyofwidths,andareusedforspansbetweenabout20and60ft.Thehollowboxbeams-andsingle-teegirdersareintendedforlongerspansuptoabout100ft.Formedium-spanhighwaybridges,toabout120ft,AASHTOstandardIbeamsaregenerallyused.Theyareintendedforusewithacompositecast-in-placeroadwayslab.Suchgirdersoftencombinepre-tensioningofthepre-castmemberwithpost-tensioningofthecompositebeamafterthedeckisplaced.Inanefforttoobtainimprovedeconomy,somestateshaveadoptedmorerefineddesigns,suchastheStateofWashingtonstandardgirders.Thespeciallydesignedpre-castgirdersmaybeusedtocarryamonorailtransitsystem.ThefinishedguidewayofWaltDisneyWorldMonorailfeaturesaseriesofsegments,eachconsistingofsixsimplysupportedpre-tensionedbeams,togethertofromacontinuousstructure.Typicalspansare100to110ft.Approximatelyhalfofthe337beamsusedhavesomecombinationofverticalandhorizontalcurvaturesandvariablesuperelevation.Allbeamsarehollow,afeatureachievedbyinsertingastyro-foamvoidinthecurvedbeamsandbyamovingmandrelinstraightbeamproduction.Pre-castgirdersmaynotbeusedforspansmuchinexcessof120ftbecauseoftheproblemsoftransportinganderectinglarge,heavyunits.Ontheotherhand,thereisacleartrendtowardtheuseoflongerspansforbridges.Forelevatedurbanexpressways,longspansfacilitateaccessandminimizeobstructiontoactivitiesbelow.Concernforenvironmentaldamagehasledtothechoiceoflongspansforcon