ConcreteBridgesConcreteisthemost-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-PERFORMANCECONCRETECompressiveStrengthFormanyyearsthedesignofprecastprestressedconcretegirderswasbasedonconcretecompressivestrengthsof34to41MPa(5,000to6,000psi).ThisstrengthlevelservedtheindustrywellandprovidedthebasisforestablishingtheprestressedconcretebridgeindustryintheUnitedStates.Inthe1990stheindustrybegantoutilizehigherconcretecompressivestrengthsindesign,andatthestartofthenewmillenniumtheindustryispoisedtoaccepttheuseofconcretecompressivestrengthsupto70MPa(10,000psi).Forthefuture,theindustryneedstoseekwaystoeffectivelyutilizeevenhigherconcretecompressivestrengths.Theready-mixedconcreteindustryhasbeenproducingconcreteswithcompressivestrengthsinexcessof70MPaforover20years.Severaldemonstrationprojectshaveillustratedthatstrengthsabove70MPacanbeachievedforprestressedconcretegirders.Barriersneedtoberemovedtoallowthegreateruseofthesematerials.Atthesametime,owners,designers,contractors,andfabricatorsneedtobemorereceptivetotheuseofhigher-compressive-strengthconcretes.DurabilityHigh-performanceconcrete(HPC)canbespecifiedashighcompressivestrength(e.g.,inprestressedgirders)orasconventionalcompressivestrengthwithimproveddurability(e.g.,incast-in-placebridgedecksandsubstructures).Thereisaneedtodevelopabetterunderstandingofalltheparametersthataffectdurability,suchasresistancetochemical,electrochemical,andenvironmentalmechanismsthatattacktheintegrityofthematerial.Significantdifferencesmightoccurinthelong-termdurabilityofadjacenttwinstructuresconstructedatthesametimeusingidenticalmaterials.Thisrevealsourlackofunderstandingandcontroloftheparametersthataffectdurability.NEWMATERIALSConcretedesignspecificationshaveinthepastfocusedprimarilyonthecompressivestrength.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.OPTIMIZEDSECTIONSInearlyapplicationsofprestressedconcretetobridges,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).SPLICEDGIRDERSSplicedconcreteI-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.SEGMENTALBRIDGESSegmentalconcretebridgeshavebecomeanestablishedtypeofconstructionforhighwayandtransitprojectsonconstrainedsites.Typicalapplicationsincludetransitsystemsoverexistingurbanstreetsandhighways,reconstructionofexistinginterchangesandbridgesundertraffic,orprojectsthatcrossenvironmentallysensitivesites.Inaddition,segmentalconstructionhasbeenprovedtobeappropriateforlarge-scale,repetitivebridgessuchaslongwaterwaycrossingsorurbanfreewayviaductsorwheretheaestheticsoftheprojectareparticularlyimportant.Currentdevelopmentssuggestthatsegmentalconstructionwillbeusedonalargernumberofprojectsinthefuture.Standardcrosssectionshavebeendevelopedtoallowforwiderapplicationofthisconstructionmethodtosmaller-scaleprojects.SurveysofexistingsegmentalbridgeshavedemonstratedthedurabilityofthisstructuretypeandsuggestthatadditionalincreasesindesignlifearepossiblewiththeuseofHPC.Segmentalbridgeswithconcretestrengthsof55MPa(8,000psi)ormorehavebeenconstructedoverthepast5years.Erectionwithoverheadequipmenthasextendedapplicationstomorecongestedurbanareas.Useofprestressedcompositesteelandconcreteinbridgesreducesthedeadweightofthesuperstructureandoffersincreasedspanlengths.LOADRATINGOFEXISTINGBRIDGESExistingbridgesarecurrentlyevaluatedbymaintainingagenciesusingworkingstress,loadfactor,orloadtestingmethods.Eachmethodgivesdifferentresults,forseveralreasons.Inordertogetnationalconsistency,FHWArequeststhatallstatesreportbridgeratingsusingtheloadfactormethod.However,thenewAASHTOLoadandResistanceFactorDesign(LRFD)bridgedesignspecificationsaredifferentfromloadfactormethod.Adiscrepancyexists,therefore,betweenbridgedesignandbridgerating.Adraftofamanualonconditionevaluationofbridges,currentlyunderdevelopmentforAASHTO,hasspecificationsforloadandresistancefactorratingofbridges.Thesespecificationsrepresentasignificantchangefromexistingones.StateswillbeaskedtocomparecurrentloadratingswiththeLRFDloadratingsusingasamplingofbridgesoverthenextyear,andadjustmentswillbeproposed.TherevisedspecificationsandcorrespondingevaluationguidelinesshouldcompletetheLRFDcycleofdesign,construction,andevaluationforthenation'sbridges.LIFE-CYCLECOSTANALYSISThegoalofdesignandmanagementofhighwaybridgesistodetermineandimplementthebestpossiblestrategythatensuresanadequatelevelofreliabilityatthelowestpossiblelife-cyclecost.Severalrecentregulatoryrequirementscallforconsiderationoflife-cyclecostanalysisforbridgeinfrastructureinvestments.Thusfar,however,theintegrationoflife-cyclecostanalysiswithstructuralreliabilityanalysishasbeenlimited.Thereisnoacceptedmethodologyfordevelopingcriteriaforlife-cyclecostdesignandanalysisofnewandexistingbridges.Issuessuchastargetreliabilitylevel,whole-lifeperformanceassessmentrules,andoptimuminspection-repair-replacementstrategiesforbridgesmustbeanalyzedandresolvedfromalife-cyclecostperspective.Toachievethisdesignandmanagementgoal,statedepartmentsoftransportationmustbegintocollectthedataneededtodeterminebridgelife-cyclecostsinasystematicmanner.Thedatamustincludeinspection,maintenance,repair,andrehabilitationexpendituresandthetimingoftheseexpenditures.Atpresent,selectedstatedepartmentsoftransportationareconsideringlife-cyclecostmethodologiesandsoftwarewiththegoalofdevelopingastandardmethodforassessingthecost-effectivenessofconcretebridges.DECKSCast-in-place(CIP)deckslabsarethepredominantmethodofdeckconstructionintheUnitedStates.Theirmainadvantageistheabilitytoprovideasmoothridingsurfacebyfield-adjustmentoftheroadwayprofileduringconcreteplacement.Inrecentyearsautomationofconcreteplacementandfinishinghasmadethissystemcost-effective.However,CIPslabshavedisadvantagesthatincludeexcessivedifferentialshrinkagewiththesupportingbeamsandslowconstruction.RecentinnovationsinbridgedeckshavefocusedonimprovementtocurrentpracticewithCIPdecksanddevelopmentofalternativesystemsthatarecost-competitive,fasttoconstruct,anddurable.Focushasbeenondevelopingmixesandcuringmethodsthatproduceperformancecharacteristicssuchasfreeze-thawresistance,highabrasionresistance,lowstiffness,andlowshrinkage,ratherthanhighstrength.Full-depthprecastpanelshavetheadvantagesofsignificantreductionofshrinkageeffectsandincreasedconstructionspeedandhavebeenusedinstateswithhightrafficvolumesfordeckreplacementprojects.NCHRPReport407onrapidreplacementofbridgedeckshasprovidedaproposedfull-depthpanelsystemwithpanelspretensionedinthetransversedirectionandposttensionedinthelongitudinaldirection.Severalstatesusestay-in-place(SIP)precastprestressedpanelscombinedwithCIPtoppingfornewstructuresaswellasfordeckreplacement.Thissystemiscost-competitivewithCIPdecks.TheSIPpanelsactasformsforthetoppingconcreteandalsoaspartofthestructuraldepthofthedeck.Thissystemcansignificantlyreduceconstructiontimebecausefieldformingisonlyneededfortheexteriorgirderoverhangs.TheSIPpanelsystemsuffersfromreflectivecracking,whichcommonlyappearsoverthepanel-to-paneljoints.AmodifiedSIPprecastpanelsystemhasrecentlybeendevelopedinNCHRPProject12-41.SUBSTRUCTURESContinuityhasincreasinglybeenusedforprecastconcretebridges.Forbridgeswithtotallengthslessthan300m(1,000ft),integralbridgeabutmentsandintegraldiaphragmsatpiersallowforsimplicityinconstructionandeliminatetheneedformaintenance-proneexpansionjoints.Althoughthemajorityofbridgesubstructurecomponentscontinuetobeconstructedfromreinforcedconcrete,prestressinghasbeenincreasinglyused.Prestressedbentsallowforlongerspans,improvingdurabilityandaestheticsandreducingconflictswithstreetsandutilitiesinurbanareas.Prestressedconcretebentsarealsobeingusedforstructuralsteelbridgestoreducetheoverallstructuredepthandincreaseverticalclearanceunderbridges.Precastconstructionhasbeenincreasinglyusedforconcretebridgesubstructurecomponents.Segmentalhollowboxpiersandprecastpiercapsallowforrapidconstructionandreduceddeadloadsonthefoundations.Precastingalsoenablestheuseofmorecomplexformsandtexturesinsubstructurecomponents,improvingtheaestheticsofbridgesinurbanandruralareas.RETAININGWALLSThedesignofearthretainingstructureshaschangeddramaticallyduringthelastcentury.Retainingwalldesignhasevolvedfromshortstonegravitysectionstoconcretestructuresintegratingnewmaterialssuchasgeosyntheticsoilreinforcementsandhigh-strengthtie-backsoilanchors.Thedesignofretainingstructureshasevolvedintothreedistinctareas.Thefirstisthetraditionalgravitydesignusingthemassofthesoilandthewalltoresistslidingandoverturningforces.Thesecondisreferredtoasmechanicallystabilizedearthdesign.Thismethodusesthebackfillsoilexclusivelyasthemasstoresistthesoilforcesbyengagingthesoilusingsteelorpolymericsoilreinforcements.Athirddesignmethodisthetie-backsoilorrockanchordesign,whichusesdiscretehigh-strengthrodsorcablesthataredrilleddeepintothesoilbehindthewalltoprovideadeadanchoragetoresistthesoilforces.Amajoradvancementintheevolutionofearthretainingstructureshasbeentheproliferationofinnovativeproprietaryretainingwalls.Manycompanieshavedevelopedmodularwalldesignsthatarehighlyadaptabletomanydesignscenarios.Theinnovativedesignscombinedwiththemodularstandardsectionsandpanelshaveledtoasignificantdecreaseinthecostforretainingwalls.Muchresearchhasbeendonetoverifythestructuralintegrityofthesesystems,andmanystateshaveembracedthesetechnologies.RESEARCHTheprimaryobjectivesforconcretebridgeresearchinthe21stcenturyaretodevelopandtestnewmaterialsthatwillenablelighter,longer,moreeconomical,andmoredurableconcretebridgestructuresandtotransferthistechnologyintothehandsofthebridgedesignersforapplication.TheHPCsdevelopedtowardtheendofthe20thcenturywouldbeenhancedbydevelopmentofmoredurablereinforcement.Inaddition,higher-strengthprestressingreinforcementcouldmoreeffectivelyutilizetheachievablehigherconcretestrengths.Lower-relaxationsteelcouldbenefitanchorzones.Also,posttensioningtendonsandcable-stayscouldbebetterdesignedforeventualrepairandreplacement.Asournaturalresourcesdiminish,theinvestigationoftheuseofrecycledmaterialsisasimportantastheresearchonnewmaterials.Thedevelopmentofmoreefficientstructuralsectionstobetterutilizetheperformancecharacteristicsofnewmaterialsisimportant.Inaddition,moreresearchisrequiredintheareasofdeckreplacementpanels,continuityregionsofsplicedgirdersections,andsafe,durable,cost-effectiveretainingwallstructures.Researchintheareasofdesignandevaluationwillcontinueintothenextmillennium.TheuseofHPCwillbefacilitatedbytheremovaloftheimpliedstrengthlimitationof70MPa(10.0ksi)andotherbarriersintheLRFDbridgedesignspecifications.Asournation’sinfrastructurecontinuestoageandasthevehicleloadscontinuetoincrease,itisimportanttobetterevaluatethecapacityofexistingstructuresandtodevelopeffectiveretrofittingtechniques.Improvedquantificationofbridgesystemreliabilityisexpectedthroughthecalibrationofsystemfactorstoassessthemembercapacitiesasafunctionofthelevelofredundancy.Dataregardinginspection,maintenance,repair,andrehabilitationexpendituresandtheirtimingmustbesystematicallycollectedandevaluatedtodevelopbettermethodsofassessingcost-effectivenessofconcretebridges.Performance-basedseismicdesignmethodswillrequireahigherlevelofcomputingandbetteranalysistools.Inbothnewandexistingstructures,itisimportanttobeabletomonitorthe“health”ofthesestructuresthroughthedevelopmentofinstrumentation(e.g.,fiberoptics)todeterminethestateofstressesandcorrosioninthemembers.CONCLUSIONIntroducedintotheUnitedStatesin1949,prestressedconcretebridgestodayrepresentover50percentofallbridgesbuilt.Thisincreasehasresultedfromadvancementsindesignandanalysisproceduresandthedevelopmentofnewbridgesystemsandimprovedmaterials.Theyear2000setsthestageforevengreateradvancements.Anexcitingfutureliesaheadforconcretebridges!混凝土橋梁在美國(guó)甚至在世界橋梁上，混凝土是最常用的建設(shè)材料。從1949年開始在賓夕法尼亞州的費(fèi)城隨著高強(qiáng)度的鋼絲應(yīng)用于核桃里橋上，橋梁預(yù)應(yīng)力應(yīng)用得到了迅速穩(wěn)步的增長(zhǎng)。從1950年到90年代初，根據(jù)美國(guó)聯(lián)邦高速公路管理局的1994年全國(guó)橋梁的庫(kù)存數(shù)據(jù)中得知，在美國(guó)超過50%已建橋梁上預(yù)應(yīng)力混凝土橋梁已經(jīng)有了很大的發(fā)展。在延長(zhǎng)混凝土橋梁跨度的能力上預(yù)應(yīng)力也發(fā)揮了重要作用。到90年代末，拼接梁跨度達(dá)到創(chuàng)紀(jì)錄的100米（330英尺）。在1974年節(jié)段混凝土橋梁的建造工程于美國(guó)開始興起。到現(xiàn)在接近200節(jié)段混凝土橋梁已建成或正在建設(shè)，其跨度達(dá)240米（800英尺）。到了70年代后期，對(duì)混凝土橋梁，斜拉橋建設(shè)開始運(yùn)用了。到1982年，在佛羅里達(dá)州的坦帕市陽(yáng)光高架橋其主跨有365米（1,200英尺），打破混凝土橋梁的新紀(jì)錄。第二年，在佛羅里達(dá)州的杰克遜維爾市達(dá)梅斯點(diǎn)大橋更新了紀(jì)錄達(dá)到400米（1300英尺）。高性能混凝土抗壓強(qiáng)度多年來，預(yù)制預(yù)應(yīng)力混凝土梁的設(shè)計(jì)是基于混凝土34至41兆帕（5000至6000PSI）的抗壓強(qiáng)度上。這種強(qiáng)度有很好的耐久性能，并提供了建設(shè)美國(guó)預(yù)應(yīng)力混凝土橋梁工業(yè)的基礎(chǔ)。在20世紀(jì)90年代該行業(yè)開始利用更高的混凝土抗壓強(qiáng)度設(shè)計(jì)值，并在新千年開始之際，業(yè)界準(zhǔn)備接受使用抗壓強(qiáng)度高達(dá)70兆帕（10,000PSI）的混凝土。對(duì)于未來，該行業(yè)需要設(shè)法有效利用更高抗壓強(qiáng)度的混凝土。超過20年預(yù)拌混凝土行業(yè)已經(jīng)能生產(chǎn)70兆帕斯卡以上混凝土抗壓強(qiáng)度。幾個(gè)示范項(xiàng)目說明，70兆帕斯卡以上的優(yōu)勢(shì)，可以促成預(yù)應(yīng)力混凝土梁實(shí)現(xiàn)。允許更多地使用這些材料來排除障礙。同時(shí)，業(yè)主，設(shè)計(jì)師，承包商和制造者必須接受更高抗壓強(qiáng)度的混凝土。耐久性高性能混凝土（HPC）可以定為抗壓強(qiáng)度高（例如，在預(yù)應(yīng)力梁）或傳統(tǒng)的耐久性抗壓強(qiáng)度（如鑄造，就地橋面和子結(jié)構(gòu)）。這就有必要對(duì)所有參數(shù)有更好了解，如抗化學(xué)，電化學(xué)耐久性，環(huán)境機(jī)制，攻擊完整的材料。顯著性差異可能發(fā)生在同一時(shí)間使用相同的材料雙相鄰結(jié)構(gòu)的長(zhǎng)期耐用性上。這揭示了我們?nèi)狈斫夂涂刂朴绊懩途眯缘膮?shù)。新材料在過去，混凝土設(shè)計(jì)規(guī)范關(guān)心的重點(diǎn)主要是抗壓強(qiáng)度?；炷潦锹呦虻囊粋€(gè)精心設(shè)計(jì)的材料，其性能可直接由設(shè)計(jì)者改變。材料特性，如透氣性，延展性，抗凍融，耐久性，耐磨損性，反應(yīng)性和強(qiáng)度都有所規(guī)定。高性能計(jì)算的倡議在促進(jìn)這些規(guī)范方面已有很長(zhǎng)時(shí)間，但還有很多可以做。添加劑，這種纖維或化學(xué)物質(zhì)，可以顯著改變混凝土的基本屬性。其他新材料，如纖維增強(qiáng)聚合物復(fù)合材料，非金屬加固（玻璃纖維增強(qiáng)和碳纖維增強(qiáng)塑料等），新金屬增援，或高強(qiáng)度鋼筋也可以用來提高這些被認(rèn)為是傳統(tǒng)的材料的性能。當(dāng)與高強(qiáng)度混凝土配合時(shí)，高強(qiáng)度鋼筋可能是特別有效。正如我們的天然資源在減少一樣，替代總來源（如再生骨料）及再生產(chǎn)品與膠凝材料替代目前正在進(jìn)一步研究。高活性水泥和聚合反應(yīng)將給與關(guān)注，就像長(zhǎng)期的耐久性新材料一樣變得司空見慣。在維修和改造方面，新材料的需求也會(huì)增加。正如已有的橋會(huì)繼續(xù)變老，那么增加結(jié)構(gòu)的壽命將變得至關(guān)重要。在改裝及維修上，一些創(chuàng)新的材料，對(duì)整個(gè)橋梁來說雖然不是經(jīng)濟(jì)的，但是不久將發(fā)現(xiàn)他的好處。截面優(yōu)化在預(yù)應(yīng)力混凝土橋梁早期應(yīng)用中，設(shè)計(jì)者開發(fā)出他們自己的最好梁段的想法。其結(jié)果是，每個(gè)承包商使用時(shí)都略有不同的梁的形狀。它太昂貴，設(shè)計(jì)者不能為每個(gè)項(xiàng)目都定制大梁。因此，對(duì)公共道路局的代表（現(xiàn)聯(lián)邦公路管理局），國(guó)家公路官員（AASHO）美國(guó)協(xié)會(huì)（現(xiàn)AASHTO標(biāo)準(zhǔn)）和預(yù)應(yīng)力混凝土協(xié)會(huì)（PCI）開始進(jìn)行標(biāo)準(zhǔn)梁部分的設(shè)計(jì)。這AASHTO-PCI標(biāo)準(zhǔn)的第一到第四類型的梁段是在50年代末產(chǎn)生的，和第五和第六類型是在六十年代初開發(fā)的。毫無疑問，標(biāo)準(zhǔn)化的梁簡(jiǎn)化了設(shè)計(jì)者的疑問，以致預(yù)應(yīng)力混凝土橋梁得到更廣泛的的使用，更重要的是導(dǎo)致成本的降低。隨著在預(yù)應(yīng)力混凝土設(shè)計(jì)和施工技術(shù)進(jìn)步，許多國(guó)家開始改善其設(shè)計(jì)和開發(fā)自己的標(biāo)準(zhǔn)部分。因此，在70年代末，聯(lián)邦公路管理局贊助的一項(xiàng)研究，以評(píng)估現(xiàn)有的標(biāo)準(zhǔn)梁段，并確定最有效的大梁。這項(xiàng)研究的結(jié)論是，氣泡似的梁是最有效的部分。在相同跨徑下，這些部分可能導(dǎo)致與AASHTO第六標(biāo)準(zhǔn)型相比減少高達(dá)百分之三十五的重量，和比AASHTO-PCI型梁相比成本節(jié)省高達(dá)百分之十七。在聯(lián)邦公路管理局研究的基礎(chǔ)上，PCI發(fā)展成PCIbulb-tee的標(biāo)準(zhǔn)型，這些是由橋梁工程師通過AASHTO標(biāo)準(zhǔn)在1987年年度會(huì)議做出的。隨后PCI–氣球型截面在幾個(gè)州被采用。此外，類似的截面產(chǎn)生了并在佛羅里達(dá)州，內(nèi)布拉斯加州和新英格蘭州被采用。此外，類似的截面是發(fā)達(dá)國(guó)家和佛羅里達(dá)州，內(nèi)布拉斯加州和新英格蘭州通過。這些高強(qiáng)度混凝土截面的梁性價(jià)比高，其跨度達(dá)60米（200英尺）。主梁拼接拼接的并且跨度在35到90米（120到300英尺）范圍內(nèi)的混凝土I-梁橋效能價(jià)格合算的。其他形狀除了I型梁包括U，T和矩形梁，雖然在迄今為止應(yīng)用中占主導(dǎo)的是I-梁，主要是因?yàn)槠涑杀鞠鄬?duì)較低。拼接橋梁的特點(diǎn)是靈活，他們?cè)诳玳L(zhǎng)，數(shù)量和橋墩位置的選擇上提供段長(zhǎng)和接頭。拼接梁通過使用短部分梁段來適應(yīng)彎曲的地方和容許在方向和接頭的變化。在拼接梁橋的連續(xù)性可以通過全長(zhǎng)后張來實(shí)現(xiàn)，傳統(tǒng)的鋼筋在甲板，高強(qiáng)度螺紋桿拼接，或預(yù)應(yīng)力鋼絞線剪接，雖然絕大多數(shù)申請(qǐng)利用全長(zhǎng)后張法來施工。抗壓強(qiáng)度高混凝土的可用性比傳統(tǒng)的高34兆帕（5000PSI）顯著提高拼接梁的設(shè)計(jì)性能，其中在靠近橋墩高彎曲部分剪切應(yīng)力會(huì)有集中。在被動(dòng)區(qū)域標(biāo)準(zhǔn)化腋梁墩部分需要提高性能。目前，這部分形狀從逐漸增厚底部凸緣變化到一個(gè)有固定大小的底部凸緣和可變深度的彎曲腰身部分。橋梁預(yù)制節(jié)預(yù)制節(jié)段混凝土橋梁已成為在限制用地建設(shè)項(xiàng)目和高速公路上制定的類型。典型的應(yīng)用包括在交通運(yùn)輸系統(tǒng)在現(xiàn)有的城市街道和公路，現(xiàn)有的交匯處和橋梁，交叉或項(xiàng)目環(huán)境敏感地點(diǎn)。此外，節(jié)段施工已被證明是可行的，如長(zhǎng)水道渡口或城市快速路高架橋或該項(xiàng)目的美學(xué)尤其重要橋梁。目前的事態(tài)發(fā)展表明，在未來節(jié)段施工將在更多的項(xiàng)目上使用。標(biāo)準(zhǔn)斷面已經(jīng)制定，以便更廣泛地在小規(guī)模的工程上運(yùn)用這些建筑方法?，F(xiàn)有段橋梁的調(diào)查表明這種結(jié)構(gòu)類型的耐用性，建議在設(shè)計(jì)使用年限與額外增加的高性能計(jì)算成為可能。在過去的5年中，混凝土的強(qiáng)度達(dá)到55兆帕斯卡（8000平方英寸）或更高的多節(jié)段橋梁已建成。設(shè)備安裝工程的開銷已擴(kuò)展應(yīng)用更擁擠的城市地區(qū)。預(yù)應(yīng)力鋼和混凝土橋梁的使用減少了上層建筑的自重，并且增加了跨度?，F(xiàn)有橋梁的額定負(fù)荷目前，維護(hù)機(jī)構(gòu)利用工作壓力，負(fù)荷率，或負(fù)載等測(cè)試方法對(duì)現(xiàn)有的橋梁進(jìn)行評(píng)估。每個(gè)方法給出不同的結(jié)果，有幾個(gè)原因。為了獲得國(guó)家的一致性，聯(lián)邦公路管理局要求所有國(guó)家的橋梁評(píng)級(jí)報(bào)告使用負(fù)載因子的方法。但是，新的AASHTO標(biāo)準(zhǔn)荷載和抗力系數(shù)設(shè)計(jì)（荷載抗力系數(shù)）的橋梁設(shè)計(jì)規(guī)范