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Unit17PrestressedConcreteBeam

BridgeConstructionTechnology

预应力混凝土梁桥施工技术

内容摘要

体外预应力目前已成为预应力体系中的重要组成部分。

但在产生初期及发展过程中，曾面临过许多问题。

本文介绍了体外预应力产生、发展及曾经历代研究和实验。

在过去的时间里，它从实验性应用的状态，发展成为德国的箱梁桥的标准设计方法，这说明体外预应力的研究已取得了阶段性的成果。

Altwipfergrund桥实例表明，仍然有许多改善桥梁体系和预应力体系的可能性。

A.Text

NewTechnologiesofPrestressed

ConcreteBredges

1Reviewofexternallyprestressedconcretebridge

Theworld’sfirstexternallyprestressedconcretebridgewasbuiltinAue,Saxonyfrom1935to1937basedonaconceptproposedbyDischinger（DRP727429）.Thebeambridgehadavariabledepth,threespans（25.2m-69.0m-23.4m）andthecentralspanconsistedofcantileverswithasimplysupportedbeaminthemiddle.Suspendedunboundedtendonsrunningoutsidetheconcretecross-sectionwereused.Howevercorrosionofthetendonsandhigherthananticipatedlossofprestresscausedbycreepandshrinkageleadtoproblemswiththisbridge.

After1945devlopmentsinbridgeconstructioninGermanywereconcentratedoninternal,post-tensionedsystems.However,externalprestressingwasusedinbothBelgiumandFrance.InGermanythecorrosionresistanceofexternaltendonswasconsideredtobeinadequateatthattime,whereaspost-tensionedbridgeswithinternaltendonsappeared

unproblematicandsoonlythisconstructionmethodwasfurtherdevelopedandapplied.

Nevertheless,thecorrosionprotectionsystemswereimprovedbythemiddleofthe1980’s.Andsoexternalprestressingseemedtobeaviablealternativetoconventionalinternalprestressing.Thismethodwasfirstusedforspecialapplications（e.g.strengthening）andlaterforstandardapplicationsasdurabilityandlong-termmaintenanceconsiderationsbecameofprimeimportance.SomepilotprojectswereconstructedofwhichthevalleybridgeRudertinginBavariawasone.Thisconstructionmethodprovedverysuccessful.ThegoodexperienceswiththeexternalprestressinginbridgeconstructionandthebenefitsinmaintenanceweethemainreasonsfortheintroductionofexternalprestressingasthestandarddesignmethodforboxgirderbridgesinGermany.

2Technology

2.1LayoutofTendons

Basicallytwodifferentlayoutsfortheexternalprestressingtendonscanbechosen.Examplesarepresentedinsection4.

1）Deflectedtendons

Thetendonsaredeflectedoverthecolumnsaswellasinthespans,e.g.atmid-apan（Fig.17-la））.Deflectionsaddlesmustbeusedinthiscase.Theshapepositionofthedeflectionsaddlesmustbecarefullycontrolled,thereforeoftenprecastconcreteelementsorsteelunitsareused.

2）Straighttendons

Thetendonsareplacedstraightandeccentricwithinthecrosssection.Twocrossgirdersorotheranchoragesystemsarenecessaryineveryspantoanchorthetendonforces（Fig.17-1b））.

2.2ConstructionMethods

Span-by-spanimplementationwithfalsework,scaffoldcarriageorincrementallaunchingwereusedinthepilotprojects.Dependentupontheconstructionmethodoneofthefollowingtendonlayoutsmaybechosen:

●Straightordeflected

●Combinationofbothlayouts

●Centric,straighttendonsfortheconstructioncanbereconstructedintoadeflectedshape（forexamplebyapplyingtheincrementallaunchingtechnique）

●Furthermoreinternalandexternalprestressingincombinationispossible（incrementallaunching）

2.3ExternalPrestressingsystems

AtthemomentseveralexternalprestressingsystemsareapprovedbytheconstructionauthoritiesforuseinthemarketinGermany.Inadditionalmosteveryproducerisdevelopingnewsystems.Soitisexpectedthatthevarietyofproductsavailableinthemarketwillexpand.

3StandardDesignMethod

Asaresultofthefavorableexperiencesgainedduringthepilotprojectsthehighwayauthoritydecidedtointroduceexternalprestressingasastandarddesignmethodforboxgirderbridges.Theauthorityexpectsthefollowingadvantages,especiallywithregardtodurabilityandmaintenance:

●Thetendonsareeasilycontrollableandverifiable

●Theyarereplaceable

●Theycanbesubjectedtoasecondarytensioning（re-stressing）

●Theyhaveahighquality,industrialcorrosionprotection

●Thestressfromtrafficvibrationissmall

●Partialprestressingcanbeutilizedforeconomicconstruction,exhibitingductilebehaviorequivalenttoalargerquantityofconventionalreinforcing

●Thewebofthesuperstructureisfreeoftendons,makingplacingoftheconcreteeasier

●Theconstructionsiteislessweatherdependentbecausenogroutingoftendonsisnecessary

InGermanyacodeforprestressingwithexternaltendonsisnotintroducedbythebuildingadministration.Thereforetheroadadministrationhasinvitedexpertstoformulateregulationswhicharebasedontheexperiencegainedfromthepilotprojectsundertaken.

4.Examples

4.1PilotProjectRuderting

In1994,thevalleybridgeRuderting,onroute85fromPassautoCham,wasdesignedasaBavarianpilotprojectusingexternalprestressing.Thisbridgehas6spans（36m﹢4×45m﹢36m）,andcrossesthevalleyHasebachtalataheightofapproximately43m.Thestraightroutingandlightcurvatureofthegradientofferedtheoptimalrepuirementsfortheuseofexternalprestressing.

AdetailedinvitationtotenderwasprepaedbythehighwayadministrationinPassauin

Co-operationwiththeauthor.Aboxgirderwaschosenforthesuperstructure,withaheightof2.7mandaregularspanof45.0m（slenderness=1/16.7）.Partialprestressingwasplanned.

Noconstructionmethodwasspecifiedinthedesign.The252mlongbridgecouldbebuiltwithfalseworkorwithincrementallaunching.Therefore,bothpossibilitieswerepre-investigatedandincludedinthedesign.Theprestressingsystemwasnotspecified.However,allprestressingsystemsshouldhavebeenapprovedbytheappropriateregulatoryauthority.

ThedraftbythehighwayadministrationwasadvertisedEurope-wide.Alternativebidswereallowedunderthefollowingconditions:

●Nochangeinspansandoveralllength

●Slendernessofsuperstructureh/L≦1/16

●Thetendonsmustbeexchangeableunderthefulltrafficloadofbridge

●Announcementoffailurethroughcrackformation

●Possibilityofanincreaseintrafficloadby20%throughstructuralprovisionsfor4additionaltendons

●Notransverseprestressing

Initally,9bidderssubmitted9mainoffersand12secondaryoffers.Onanalyzingtheoffers,aspecialofferfromthecontractorBilfinger&BergerBauAGwasthemosteconomicalandsoobtainedthecontract.

Thisspecialofferchosestraighttendonsforprestressingthe2.60mhighboxgirder.Therangeofthespansandunderthedeckslabintherangeofthecolumns.Theanchoringofthesetendonswasperformedintheopeningofthespancrossgirders,whichwereplaced8minfrontandbehindhecolumncrossgirders.Theremainingtendonswereplacedatthecentroldofthecrosssectionareaandareonlyanchoredinthefinalstateintheendcrossgirders.Duetotheslightsagofthebridge,Thecontinuous,Coupledtendonshadtobedeflectedattwosaddles.Theconstructionofthesuperstructurefollowedinthreesectionsonfalsework.

4.2ValleyBridgeTrockau

Therenovationofthe60yearsoldhighwayA9,themostimportantconnectionbetweenBavariaandthenewly-formedGermanstates,necessitatesthisnewbridgebuilding.Thebiddingforthe6lanehighwaybridgewasperformedbythelocalhighwayauthorityintheyear1998.Therebyacompositebridgewith9spans（48.0﹢58.0﹢60.0﹢85.0﹢100.0﹢85.0﹢60.0﹢58.0﹢48.0m）wasplaned.

Howeverconcreteboxgirderswithexternalprestressingwereproposedinsomeredesign.Aworkinggroup-consistingofMaxB.gl,Neumarkt,andWalterBau-AG,NiederlassungNurnberg-wasinstructedtobuildthebridgewithonlyexternalprestressing,asitwasproposedintheirredesign.Deflectedtendonswerechosenaswellas

Straighttendonsinthecolumnareasandmiddlefieldareas.Scaffoldcarriagewasusedasconstructionsystem.Thesuperstructureconsistsoftwoseparatedsingleboxgirderswithavariabledepthbetween3.5and6.5mintheareaofthelargecentralspan.

4.3HybridConstruction:

ValleyBridgeAltwipfergund

Thereductionofdeadloadleadtothisnewconstructionmethod,whichiscalledhybridconstruction.Theaimistotakeadvantageofthedifferentmaterials.Theconcreteisusedforthedeckandthebottomslab.Prestressingisintroducedinsectionswithtensilestresses.Thewebshaveonlytoconnectthetwopointcrosssectionandtotransferthetransverseforces.Forthisafoldedsteelplateisused.Withthisgeometrythewebhasnearlynolongitudinalstiffness,butbecauseofthehighertransversestiffnessthetransverseforcescanbetransferred.

Oneproblemthatoccurredwiththisconstructionis,thatthedeformationscausedbythetransverseforcesarenotnegligible.Thesedeformationsstronglydependontheimperfectionsofthesteelweb.Thereforeaexperimentononesectionoftheweb（height:

4.5m,length:

9.0m）forthevalleybridgeAltwipfergrundisperformedattheLehrstrhlfurMassivbauoftheTechnischenUniversitatMUnchen.Theaimofthetestistoverifytheloadcarryingbehaviorofthesefoldedwebs.

5Conclusions

Externalprestressinggetsmoreandmoreimportantforthedesignofnewbridges.Duringthelastyearsithasevolvedfromthestatusofpilotapplications（Ruderting）toastandarddesignmethodforboxgirdersinGermany（Trockau）.AndastheexampleAltwipfergrundshows,therearestillpossibilitiestoimprovethebridgesystemsandtheprestressingsystems.

NewWordsandExpressions

Prestressedconcrete预应力混凝土

Cantilevern.悬臂

Simply简支

Tendonn.刚束

Cross-section横截面

Corrosionn.锈蚀

Lossofprestress预应力损失

Creepn.徐变

ShrinkageN收缩

Post-tensionedmethod后张法

Externalprestressing体外预应力

Unit18BridgeRehabilitati