建筑 土木工程 外文翻译 外文文献 英文文献 混凝土桥梁doc.docx

建筑 土木工程 外文翻译 外文文献 英文文献 混凝土桥梁doc.docx

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建筑土木工程外文翻译外文文献英文文献混凝土桥梁doc

ConcreteBridges

Concreteisthemost-usedconstructionmaterialforbridgesintheUnitedStates,andindeedintheworld.Theapplicationofprestressingtobridgeshasgrownrapidlyandsteadily,beginningin1949withhigh-strengthsteelwiresintheWalnutLaneBridgeinPhiladelphia,Pennsylvania.AccordingtotheFederalHighwayAdministration’s1994NationalBridgeInventorydata,from1950totheearly1990s,prestressedconcretebridgeshavegonefrombeingvirtuallynonexistenttorepresentingover50percentofallbridgesbuiltintheUnitedStates.

Prestressinghasalsoplayedanimportantroleinextendingthespancapabilityofconcretebridges.Bythelate1990s,spliced-girderspansreachedarecord100m（330ft）.ConstructionofsegmentalconcretebridgesbeganintheUnitedStatesin1974.Curretly,closeto200segmentalconcretebridgeshavebeenbuiltorareunderconstruction,withspansupto240m（800ft）.

Lateinthe1970s,cable-stayedconstructionraisedthebarforconcretebridges.By1982,theSunshineSkywayBridgeinTampa,Florida,hadsetanewrecordforconcretebridges,withamainspanof365m（1,200ft）.Thenextyear,theDamesPointBridgeinJacksonville,Florida,extendedtherecordto400m（1,300ft）.

HIGH-PERFORMANCECONCRETE

CompressiveStrength

Formanyyearsthedesignofprecastprestressedconcretegirderswasbasedonconcretecompressivestrengthsof34to41MPa（5,000to6,000psi）.ThisstrengthlevelservedtheindustrywellandprovidedthebasisforestablishingtheprestressedconcretebridgeindustryintheUnitedStates.Inthe1990stheindustrybegantoutilizehigherconcretecompressivestrengthsindesign,andatthestartofthenewmillenniumtheindustryispoisedtoaccepttheuseofconcretecompressivestrengthsupto70MPa（10,000psi）.

Forthefuture,theindustryneedstoseekwaystoeffectivelyutilizeevenhigherconcretecompressivestrengths.Theready-mixedconcreteindustryhasbeenproducingconcreteswithcompressivestrengthsinexcessof70MPaforover20years.Severaldemonstrationprojectshaveillustratedthatstrengthsabove70MPacanbeachievedforprestressedconcretegirders.Barriersneedtoberemovedtoallowthegreateruseofthesematerials.Atthesametime,owners,designers,contractors,andfabricatorsneedtobemorereceptivetotheuseofhigher-compressive-strengthconcretes.

Durability

High-performanceconcrete（HPC）canbespecifiedashighcompressivestrength（e.g.,inprestressedgirders）orasconventionalcompressivestrengthwithimproveddurability（e.g.,incast-in-placebridgedecksandsubstructures）.Thereisaneedtodevelopabetterunderstandingofalltheparametersthataffectdurability,suchasresistancetochemical,electrochemical,andenvironmentalmechanismsthatattacktheintegrityofthematerial.Significantdifferencesmightoccurinthelong-termdurabilityofadjacenttwinstructuresconstructedatthesametimeusingidenticalmaterials.Thisrevealsourlackofunderstandingandcontroloftheparametersthataffectdurability.

NEWMATERIALS

Concretedesignspecificationshaveinthepastfocusedprimarilyonthecompressivestrength.Concreteisslowlymovingtowardanengineeredmaterialwhosedirectperformancecanbealteredbythedesigner.Materialpropertiessuchaspermeability,ductility,freeze-thawresistance,durability,abrasionresistance,reactivity,andstrengthwillbespecified.TheHPCinitiativehasgonealongwayinpromotingthesespecifications,butmuchmorecanbedone.Additives,suchafibersorchemicals,cansignificantlyalterthebasicpropertiesofconcrete.Othernewmaterials,suchasfiber-reinforcedpolymercomposites,nonmetallicreinforcement（glassfiber-reinforcedandcarbonfiber-reinforcedplastic,etc.）,newmetallicreinforcements,orhigh-strengthsteelreinforcementcanalsobeusedtoenhancetheperformanceofwhatisconsideredtobeatraditionalmaterial.Higher-strengthreinforcementcouldbeparticularlyusefulwhencoupledwithhigh-strengthconcrete.Asournaturalresourcesdiminish,alternativeaggregatesources（e.g.,recycledaggregate）andfurtherreplacementofcementitiousmaterialswithrecycledproductsarebeingexamined.Highlyreactivecementsandreactiveaggregateswillbeconcernsofthepastasnewmaterialswithlong-termdurabilitybecomecommonplace.

Newmaterialswillalsofindincreasingdemandinrepairandretrofitting.Asthebridgeinventorycontinuestogetolder,increasingtheusablelifeofstructureswillbecomecritical.Someinnovativematerials,althoughnoteconomicalforcompletebridges,willfindtheirnicheinretrofitandrepair.

OPTIMIZEDSECTIONS

Inearlyapplicationsofprestressedconcretetobridges,designersdevelopedtheirownideasofthebestgirdersections.Theresultisthateachcontractorusedslightlydifferentgirdershapes.Itwastooexpensivetodesigncustomgirdersforeachproject.

Asaresult,representativesfortheBureauofPublicRoads（nowFHWA）,theAmericanAssociationofStateHighwayOfficials（AASHO）（nowAASHTO）,andthePrestressedConcreteInstitute（PCI）beganworktostandardizebridgegirdersections.TheAASHTO-PCIstandardgirdersectionsTypesIthroughIVweredevelopedinthelate1950sandTypesVandVIintheearly1960s.Thereisnodoubtthatstandardizationofgirdershassimplifieddesign,hasledtowiderutilizationofprestressedconcreteforbridges,and,moreimportantly,hasledtoreductionincost.

Withadvancementsinthetechnologyofprestressedconcretedesignandconstruction,numerousstatesstartedtorefinetheirdesignsandtodeveloptheirownstandardsections.Asaresult,inthelate1970s,FHWAsponsoredastudytoevaluateexistingstandardgirdersectionsanddeterminethemostefficientgirders.Thisstudyconcludedthatbulb-teeswerethemostefficientsections.Thesesectionscouldleadtoreductioningirderweightsofupto35percentcomparedwiththeAASHTOTypeVIandcostsavingsupto17percentcomparedwiththeAASHTO-PCIgirders,forequalspancapability.OnthebasisoftheFHWAstudy,PCIdevelopedthePCIbulb-teestandard,whichwasendorsedbybridgeengineersatthe1987AASHTOannualmeeting.Subsequently,thePCIbulb-teecrosssectionwasadoptedinseveralstates.Inaddition,similarcrosssectionsweredevelopedandadoptedinFlorida,Nebraska,andtheNewEnglandstates.Thesecrosssectionsarealsocost-effectivewithhigh-strengthconcretesforspanlengthsuptoabout60m（200ft）.

SPLICEDGIRDERS

SplicedconcreteI-girderbridgesarecost-effectiveforaspanrangeof35to90m（120to300ft）.OthershapesbesidesI-girdersincludeU,T,andrectangulargirders,althoughthedominantshapeinapplicationstodatehasbeentheI-girder,primarilybecauseofitsrelativelylowcost.Afeatureofsplicedbridgesistheflexibilitytheyprovideinselectionofspanlength,numberandlocationsofpiers,segmentlengths,andsplicelocations.Splicedgirdershavetheabilitytoadapttocurvedsuperstructurealignmentsbyutilizingshortsegmentlengthsandaccommodatingthechangeindirectioninthecast-in-placejoints.Continuityinsplicedgirderbridgescanbeachievedthroughfull-lengthposttensioning,conventionalreinforcementinthedeck,high-strengththreadedbarsplicing,orpretensionedstrandsplicing,althoughthegreatmajorityofapplicationsutilizefull-lengthposttensioning.Theavailabilityofconcretecompressivestrengthshigherthanthetraditional34MPa（5,000psi）significantlyimprovestheeconomyofsplicedgirderdesigns,inwhichhighflexuralandshearstressesareconcentratednearthepiers.Developmentofstandardizedhaunchedgirderpiersegmentsisneededforefficiencyinnegative-momentzones.Currently,thesegmentshapesvaryfromagraduallythickeningbottomflangetoacurvedhaunchwithconstant-sizedbottomflangeandvariablewebdepth.

SEGMENTALBRIDGES

Segmentalconcretebridgeshavebecomeanestablishedtypeofconstructionforhighwayandtransitprojectsonconstrainedsites.Typicalapplicationsincludetransitsystemsoverexistingurbanstreetsandhighways,reconstructionofexistinginterchangesandbridgesundertraffic,orprojectsthatcrossenvironmentallysensitivesites.Inaddition,segmentalconstructionhasbeenprovedtobeappropriateforlarge-scale,repetitivebridgessuchaslongwaterwaycrossingsorurbanfreewayviaductsorwheretheaestheticsoftheprojectareparticularlyimportant.

Currentdevelopmentssuggestthatsegmentalconstructionwillbeusedonalargernumberofprojectsinthefuture.Standardcrosssectionshavebeendevelopedtoallowforwiderapplicationofthisconstructionmethodtosmaller-scaleprojects.SurveysofexistingsegmentalbridgeshavedemonstratedthedurabilityofthisstructuretypeandsuggestthatadditionalincreasesindesignlifearepossiblewiththeuseofHPC.Segmentalbridgeswithconcretestrengthsof55MPa（8,000psi）ormorehavebeenconstructedoverthepast5years.Erectionwithoverheadequipmenthasextendedapplicationstomorecongestedurbanareas.Useofprestressedcompositesteelandconcreteinbridgesreducesthedeadweightofthesuperstructureandoffersincreasedspanlengths.

LOADRATINGOFEXISTINGBRIDGES

Existingbridgesarecurrentlyevaluatedbymaintainingagenciesusingworkingstress,loadfactor,orloadtestingmethods.Eachmethodgivesdifferentresults,forseveralreasons.Inordertogetnationalconsistency,FHWArequeststhatallstatesreportbridgeratingsusingtheloadfactormethod.However,thenewAASHTOLoadandResistanceFactorDesign（LRFD）bridgedesignspecificationsaredifferentfromloadfactormethod.Adiscrepanc