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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,withamainspanofm,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,andawidthofm,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,ormilliradians(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,atthepresenttimeinclinometersandthermalsensors.Thecomputerprogramdrivingalltheinstrumentationoffersaflexib