桥梁外文文献讲课教案.docx

桥梁外文文献讲课教案.docx

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桥梁外文文献讲课教案

AUTOMATICDEFLECTIONANDTEMPERATUREMONITORINGOF

ABALANCEDCANTILEVERCONCRETEBRIDGE

byOlivierBURDET,Ph.D.

SwissFederalInstituteofTechnology,Lausanne,Switzerland

InstituteofReinforcedandPrestressedConcreteSUMMARY

Thereisaneedforreliablemonitoringsystemstofollowtheevolutionofthebehaviorofstructuresovertime.

Deflectionsandrotationsarevaluesthatreflecttheoverallstructurebehavior.Thispaperpresentsaninnovativeapproachtothemeasurementoflong-termdeformationsofbridgesbyuseofinclinometers.Highprecisionelectronicinclinometerscanbeusedtofolloweffectivelylong-termrotationswithoutdisruptionofthetraffic.Inadditiontotheiraccuracy,theseinstrumentshaveproventobesufficientlystableovertimeandreliableforfieldconditions.TheMentuebridgesaretwin565mlongbox-girderpost-tensionedconcretehighwaybridgesunderconstructioninSwitzerland.Thebridgesarebuiltbythebalancedcantilevermethodoveradeepvalley.Thepiersare100mhighandthemainspanis150m.Acentralizeddataacquisitionsystemwasinstalledinonebridgeduringitsconstructionin1997.Everyminute,thesystemrecordstherotationandtemperatureatanumberofmeasuringpoints.Thesimultaneousmeasurementofrotationsandconcretetemperatureatseverallocationsgivesaclearideaofthemovementsinducedbythermalconditions.Thesystemwillbeusedincombinationwithahydrostaticlevelingsetuptofollowthelong-termbehaviorofthebridge.Preliminaryresultsshowthatthesystemperformsreliablyandthattheaccuracyofthesensorsisexcellent.

Comparisonoftheevolutionofrotationsandtemperatureindicatethatthestructurerespondstochangesinairtemperatureratherquickly.

1.BACKGROUND

Allovertheworld,thenumberofstructuresinservicekeepsincreasing.Withthedevelopmentoftrafficandtheincreaseddependenceonreliabletransportation,itisbecomingmoreandmorenecessarytoforeseeandanticipatethedeteriorationofstructures.Inparticular,

forstructuresthatarepartofmajortransportationsystems,rehabilitationworksneedtobecarefullyplannedinordertominimizedisruptionsoftraffic.Automaticmonitoringofstructuresisthusrapidlydeveloping.

Long-termmonitoringofbridgesisanimportantpartofthisoverallefforttoattempttominimizeboththeimpactandthecostofmaintenanceandrehabilitationworkofmajorstructures.Byknowingtherateofdeteriorationofagivenstructure,theengineerisabletoanticipateandadequatelydefinethetimingofrequiredinterventions.Conversely,interventionscanbedelayeduntiltheconditionofthestructurerequiresthem,withoutreducingtheoverallsafetyofthestructure.

Thepaperpresentsaninnovativeapproachtothemeasurementoflong-termbridgedeformations.Theuseofhighprecisioninclinometerspermitsaneffective,accurateandunobtrusivefollowingofthelong-termrotations.Themeasurementscanbeperformedundertrafficconditions.Simultaneousmeasurementofthetemperatureatseverallocationsgivesaclearideaofthemovementsinducedbythermalconditionsandthoseinducedbycreepandshrinkage.ThesystempresentedisoperationalsinceAugust1997intheMentuebridge,currentlyunderconstructioninSwitzerland.Thestructurehasamainspanof150mandpiers100mhigh.

2.LONG-TERMMONITORINGOFBRIDGES

AspartofitsresearchandserviceactivitieswithintheSwissFederalInstituteofTechnologyinLausanne（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,are

shownalongwithmeasurementsperformedoveraperiodof24hours.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].

3.MENTUEBRIDGES

TheMentuebridgesaretwinbox-girderbridgesthatwillcarrythefutureA1motorwayfromLausannetoBern.Eachbridge,similarindesign,hasanoveralllengthofapproximately565m,andawidthof13.46m,designedtocarrytwolanesoftrafficandanemergencylane.Thebridgescrossadeepvalleywithsteepsides（fig.2）.Thebalancedcantileverdesignresultsfromabridgecompetition.The100mhighconcretepierswerebuiltusingclimbingformwork,afterwhichtheconstructionofthebalancedcantileverstarted（fig.3）.

4.INCLINOMETERS

Startingin1995,IBAPinitiatedaresearchprojectwiththegoalofinvestigatingthefeasibilityofameasurementsystemusinginclinometers.Preliminaryresultsindicatedthatinclinometersofferseveraladvantagesfortheautomaticmonitoringofstructures.Table1summarizesthemainpropertiesoftheinclinometersselectedforthisstudy.

Oneinterestingpropertyofmeasuringastructure’srotations,isthat,foragivenratioofmaximumdeflectiontospanlength,themaximumrotationisessentiallyindependentfromitsstaticsystem[8].Sincemaximalallowablevaluesofabout1/1,000forlong-termdeflectionsunderpermanentloadsaregenerallyacceptedvaluesworldwide,developmentsmadeforbox-girderbridgeswithlongspans,asisthecaseforthisresearch,areapplicabletootherbridges,forinstancebridgeswithshorterspansandothertypesofcross-sections.Thisissignificantbecauseoftheneedtomonitorsmallerspanswhichconstitutethemajorityofall

bridges.

TheselectedinclinometersareoftypeWylerZerotronic±1°[9].Theiraccuracyis1microradian（μrad）,whichcorrespondstoarotationofonemillimeterperkilometer,averysmallvalue.Foranintermediatespanofacontinuousbeamwithaconstantdepth,amid-spandeflectionof1/20,000wouldinduceamaximumrotationofabout150μrad,or0.15milliradians（mrad）.

Onepotentialproblemwithelectronicinstrumentsisthattheirmeasurementsmaydriftover

time.Toquantifyandcontrolthisproblem,amechanicaldevicewasdesignedallowingtheinclinometerstobepreciselyrotatedof180°inanhorizontalplane（fig.4）.Thedriftofeachinclinometercanbeverysimplyobtainedbycomparingthevaluesobtainedintheinitialandrotatedpositionwithpreviouslyobtainedvalues.Sofar,ithasbeenobservedthatthetypeof

inclinometerusedinthisprojectisnotverysensitivetodrifting.

5.INSTRUMENTATIONOFTHEMENTUEBRIDGES

Becauseanumberofbridgesbuiltbythebalancedcantilevermethodhaveshownanunsatisfactorybehaviorinservice[2,7,10],itwasdecidedtocarefullymonitortheevolutionofthedeformationsoftheMentuebridges.Thesebridgesweredesignedtakingintoconsiderationrecentrecommendationsforthechoiceoftheamountofposttensioning

[7,10,13].Monitoringstartingduringtheconstructionin1997andwillbepursuedafterthebridgesareopenedtotrafficin2001.Deflectionmonitoringincludestopographiclevelingbythehighwayauthorities,anhydrostaticlevelingsystemovertheentirelengthofbothbridgesandanetworkofinclinometersinthemainspanoftheNorthbridge.Datacollectionis

coordinatedbytheengineerofrecord,tofacilitatecomparisonofmeasuredvalues.Theinformationgainedfromtheseobservationswillbeusedtofurtherenhancethedesigncriteriaforthattypeofbridge,especiallywithregardtotheamountofpost-tensioning[7,10,11,12,13].

Theautomaticmonitoringsystemisdrivenbyadataacquisitionprogramthatgathersandstoresthedata.Thissystemisabletocontrolvarioustypesofsensorssimultaneously,atthepresenttimeinclinometersandthermal