桥梁外文文献.docx
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桥梁外文文献.docx
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桥梁外文文献
桥梁外文文献
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.
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,atthepresenttimeinclinometersandthermalsensors.Thecomputerprogramdrivingalltheinstrumentationoffersaflexibleframework,allowingthelateradditionofnewsensorsordataacquisitionsystems.TheuseofthedevelopmentenvironmentLabView[14]allowedtoleveragethelargeuserbaseinthefieldoflaboratoryinstrumentationanddataanalysis.Thedataacquisitionsystemrunsonarathermodestcomputer,withanIntel486/66Mhzprocessor,16MBofmemoryanda500MBharddisk,runningWindowsNT.Allsensordataaregatheredonceperminuteandstoredincompressedformontheharddisk.Thesystemislocatedinthebox-girderontopofpier3(fig.5).Itcanwithstandsevereweatherconditionsandwillrestartitselfautomaticallyafterapoweroutage,whichhappenedfrequentlyduringconstruction.
6.SENSORS
Figure5(a)showsthelocationoftheinclinometersinthemainspanoftheNorthbridge.Thesensorsareplacedattheaxisofthesupports(①and⑤),at1/4and3/4(③and④)ofthespanandat1/8ofthespanfor②.Inthecrosssection,thesensorsarelocatedontheNorthweb,ataheightcorrespondingtothecenterofgravityofthesection(fig.5a).Thesensorsareallconnectedbyasingle RS-485 cabletothecentraldataacquisitionsystemlocatedinthevicinityofinclinometer①.Monitoringofthebridgestartedalreadyduringitsconstruction.Inclinometers①,②and③wereinstalledbeforethespanwascompleted.Theresultingmeasurementweredifficulttointerpret,however,becauseofthewidevariationsofanglesinducedbythevariousstagesofthisparticularmethodofconstruction.
Thedeflectedshapewillbedeterminedbyintegratingthemeasuredrotationsalongthelengthofthebridge(fig.5b).Althoughthisintegrationisinprinciplestraightforward,ithasbeenshown[8,16]thatthetypeofloadingandpossiblemeasurementerrorsneedtobecarefully
takenintoaccount.
Thermalsensorswereembeddedinconcretesothattemperatureeffectscouldbetakenintoaccountfortheadjustmentofthegeometryoftheformworkforsubsequentcasts.Figure6showsthelayoutofthermalsensorsinthemainspan.Themeasurementsectionsarelocatedatthesamesectionsthantheinclinometers(fig.5).Allsensorswereplacedintheformworkbeforeconcretingandwereoperationalassoonastheformworkwasremoved,whichwasrequiredfortheneedsoftheconstruction.Ineachsection,sevenoftheninethermalsensor(indicatedinsolidblackinfig.6)arenowautomaticallymeasuredbythecentraldataacquisitionsystem.
7.RESULTS
Figure7showstheresultsofinclinometrymeasurementsperformedfromtheendof
SeptembertothethirdweekofNovember1997.Allinclinometersperformedwellduring
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