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BridgesandtheBridgeConstructionTechniques
Part1.Bridges
SimpleTrussBridge
FormanyyearsAmericanbirdge-desigersexercisedtheiringenuityindevisingnewformsoftrussesandgirders,theprincipalobjectoftheirendeavorsbeingtofindformsinvolvingtheuseofthesmallestamountofmetal.Eachformasitappearedwastestedbysubjectingittotheordealofactualuse,whichshowedconclusivelybothitsmeritsanditsdefects;hence,byaprocessofelimination,basedupontheprincipleofthesurvivalofthefittest,afewformshavebeenretainedandothershabebeenrelegatedtothehistoryofbridge-buiding.Asmighthavebeenanticipated,thefewformswhichhavesurvivedarethesimplestofall;andalthoughevenatthepresenttimeonehearsoccasionallyofsomeimprovedformoftruss,theassumedimprovementrarelymaterializes.TheformsofturssthathavebestsurvivedthetestoftimearethePratt,Petit,andmanyothers:
ThePratttruss,isthetypemostcommonlyusedinAmericaforspansunder250feetinlength.Itsadvantagesaresimplicity,economonlyofmetal,andsuitabilityforconnectingtothefloorandlateralsystems.
ThePetittruss,isamodificationofthePratt,andisgenerallyusedforspansexceeding250-300feet.
Itiscomparativelysimple,and,likethePratttruss,itiseconomicalofmetalandlendsitselfreadilytotheconnectionofthefloorandlateralsystems.
CantileverBridge
Acantileverbridgeistheonewhosespanissupportedbycantileverswhichprojectfromthepiersonwhichitrestsandwhichmeetinthecenterofthespan,wheretheyjoinedtogether.Initsdesignthecantilevertakesvariousformsbutallofthesedependontheprincipleofbalanceaboutacommoncenter.
Therearetwomainadvantagesofthisformofbridge.First,withthismethodofbridgebuilding,itispossibletousesmallerandmorecompactpiersforaviaductthanwouldbethecaseifeachpierhadtocarrytheendsoftwoadjacentgirderspans.Secondly,whenarailwayrunsacrossawidewaterwayofwhichthedepthprecludesthesinkingofmanyoranyfoundationsforintermediatepiers,thisformofbridgemaybeadopter.
However,cantileverbridgesshouldneverbeadoptedunlesstheabove-mentioned
conditionsexist,becausetheyareinfeiorinrigiditytosimpleturssbridgesandusuallyrequiremoremetalfortheirconstruction.
ArchBridge
Fordeepgorgeswithrockysides,orforshallowstreamswithrockbottomandnaturalabutments,archesanreminentlyproperandeconomical.
Theadvantagesofthearchareapossibleeconomyincostofmetalandanaestheticappearance,whileitsdiaadvantagesarealackofrigidityand,formosttypes,anduncertainyconcerningthestressesinthemembers.
Whenbridgesfoundationshavetobebuiltonpilesoronanyothermaterialthatisliabletoslightsettlement,orwhentheabutmentscouldpossiblymovelaterallyevenameretrifle,itisnotpropertoadoptanarchsuperstructure;formaysettlementoranymotionwhatsoeverineitherpiersorabutmentswouldupsettheconditionsassumedforthecomputations,andthuscausetobeincreasedtoanrncertainamountsomeofthestressesforwhichthesuperstructurewasproportions.
Cable-StayedBridge
Duringthepastdecadecable-stayedbridgeshavefoundwideapplication,especiallyinWesternEurope,andtolesserextentinotherpartsoftheworle.Cable-stayedbridgesareconstructedalongastructuralsystemwhichconprisesanorthotropicdeckandcontinuousgirderswhicharesupportedbystays,i.einclinedcablespassingoverorattachedtotowerslocatedatthemainpiers.
Theideaofusingcablestosupportbridgespansisbynomeansnew,andanumberofexamplesofthistypeofconstructionwererecordedalongtimeago.Unfortunately,thesystemingeneralmetwithlittlesuccess,duetothefacethatthestaticswerenotfullyunderstoodandthatunsuitablematerialssuchasbarsandchainswereusedtoformtheinclinedsupportsofstays.
Wideandsuccessfulapplicationofcable-stayedsystemswasrealizedonlyrecently,withtheintroductionofhigh-strengthsteels,orthotropictypedecks,developmentofweldingtechniquesandprogressinstructualanalysis.Thedevelopmentandapplicationofelectroniccomputersopenedupnewandpracticallyunlimitedpossibilitiesfortheexactsolutionofthesehighlystaticallyindeterminatesystemsandforprecisestaticalanalysisoftheirthree-dimensionalperformance.
Theintroductionofthecable-stayedsysteminbridgeengineeringhasresultedinthecreationofnewtypesofstructureswhichpossesmanyexcellentcharacteristicsandadvantages.Outstandingamongthesearetheirstructuralcharacteristics,effciencyandwiderangeofapplication.
Thebasicstructuralcharacteristicsandreasonsfortherapiddevelopmentandsuccessofcable-stayedbridgesareasoutlinedbelow.
Cable-stayedbridgespresentaspacesystem,consistingofstiffeninggirders,steelorconcretedeckandsupportingpartsastowersactinginconpressionandinclinedcablesintension.Bytheirstructuralbehaviorcable-stayedsystemsoccupyamiddlepossitionbetweenthegirdertypeandsuspensiontypebridges.
Themainstructuralcharacteristicofthissystemistheintegralactionofthestiffeninggirdersandprestressedorpost-tensionedinclinedcables,whichrunfromthetowertopsdowntotheanchorpointsatthestiffeninggirders.Horizontalcompressiveforcesduetothecableactionaretakenbygirdersangdnomassiveanchoragesarerequired.Thesubstrucrure,threrfore,isveryeconomical.
Introductionoftheorthotropicsystemhasresultedinthecreationofnewtypesofsuperstructurewhichcaneasilycarrythehorizontalthrustofstaycableswithalmostnoadditionalmaterial,evenforverylongspans.
Inoldtypesofconventionalsuperstructuresthealab,stringers,floorbeamsandmaingirderswereconsideredasactingindependently,Suchsuper-structureswerenotsuitableforcable-stayedbridges.Withtheorthotropictypedeck,however,thestiffenedplatewithitslargecross-sectionalareaactsnotonlyastheupperchordofthemaingirdersbutalsoasthehorizontalplategeideragainstwindforces,givingmodernbridgesmuchmorela-teralstiffnessthanthewindbracingusedinoldsystems,Infact,inrothotropicsystems,allelementsoftheroadwayandsecondarypartsofthesuperstructurepraticipateintheworkofmainbridgesystem.Thisresultsinreductionofthedepthofthegirdersandeconomyinthesteel.
Anotherstructuralcharacteristicofthissystemisthatitiasgeometricallyunchangeableunderanyloadpositiononthebridge,andallcablesarealwaysinastateoftension.Thischaracteristicofthecable-stayedsystemspermitthentobebuiltfromrelativelylightflexibleelements-cables.
Theimportantcharacteristicsofsuchathree-dimensionalbridgeisthefullparticipationofthetransversestructuralpartintheworkofthemainstructureinthelongitudinaldirection.Thismeansaconsiderableincreaseinthemonmentoftheinertiaoftheconstruction,whichpermitsareductionofthedepthofthegirdersandaconsequentsavinginsteel.
Theorthotropicsystemprovidesthecontinuityofthedeckstructureatthetowersandintheventerofthemainapan.Thecontinuityofthebridgesuperstructureovermanyspanshasmanyadvantagesandisactuallynecessaryforagoodcable-stayedbridge.
Part2.BridgeConstructionTechniques
Thefinalcostofabridgeisthesumofthecostofpermanentmaterials,theproportionatecosttotheprojectofplantandtemporaryworksandthecostoflabor.Thecostofpermanentmaterialscanbeestimatedreasonablycorrectly.Withexperience,abridgecontractorcandealcompletelywithcostofplantandtemporaryworks.Butthelaborcostdoesnotlenditselftoexactanalysis.Recentcompetitivedesignshaveattemptedtointroduceinnovationsinconstructionmethodswithaviewtoeffecteconomyinthecostonlaborbyreducingtemporaryworksandbyminimizingthedurationofsitework.
Thesuitabletechniquesofconstructionofbridgesuperstructurewillvaryfromsitetosite,andwilldependonthespansandlengthofthebridge,typeofthebridge,materialsusedandsiteconditions.Forinstance,cast-in-siteconcreteconstructioncouldbeadoptedforshortspansupto40m,iftheriverbedisdryforaconsiderateportionoftheyear,whereasfreecantileverconstructionwithprestressedconcretedeckingwouldbeappropriateforlongspansinriverswithnavigationalrequirements.Thecurrenttrendistowardstheavoidanceofstagingasmuchaspossibleandtouseprecastorprefabricatedcomponentstomaximumextent.Also,constructionmachinerysuchascranesandlaunchinggirdersarecomingintowideruse.Thesearegreatersavingstobeeffectedbypayingattentiontothemethodofconstructionevenfromthedesignstagethanbyattackingpermanentmaterials.
ShortSpanBridges
Forbridgesinvolvingspansupto40m,thesuperstructuremaybebuiltonstagingsupportedontheground.Alternatively,thegirdersmaybeprecastforthefullspanlengthanderectedusinglaunchinggirdersorcranes,ifthebridgehasmanyequalspans.Inthelatterprocedure,theadditionalcostonerectionequipmentshouldbelessthanthesavinginthecostofformworkandinthelabourcostresultingfromfasterconstruction.
LongSpanConcreteBridges
Longspanconcretebridgesareusuallyofpost-tensionedconcreteandconstructedeitherasconditionsbeamstypesorasfreevercantilestructures.Manymethodshavebeendevelopedforcontinuousdeckconstruction.Iftheclearancebetweenthegroundandbottomofthedeckissmallandthesoilisfirm,thesuperstructurecanbebuiltonstaging.Thismethodisbecomingobsolete.Currently,free-cantileverandmovablescaffoldsystemsareincreasinglyusedtosavetimeandimprovesafety.
Themovablescaffoldsystememploysmovableformsstiffenedbysteelframes.Theseformsextendonespanlengthandaresupportedbysteelgirderswhichrestonapieratoneendandcanbemovedfromspantospanonasecondsetofauxiliarysteelgirders.
Aneconomicalconstructiontechniqueknownasincrementalpush-launchingmethoddevelopedbyBaur-LeonhardteamisshownschematicallyinFigure22.1.
Thetotalcontinuousdeckissubdividedlongitudinallyintosegmentsof10to30mlengthdependingonthelengthofspansandthetimeavailableforconstruction.Eachofthesesegmentsisconstructedimmediatelybehindtheabutmentofthebridgeinsteelframedforms,whichremaininthesameplaceforconcretingallsegments.Theformsaresodesignedastobecapableofbeingmovedtransverselyorrotatedonhingestofacilitateeasystrippingaftersufficienthardeningofconcrete.Attheheadofthefirstsegment,asteelnoseconsistingofalighttrussisattachedtofacilitatereachingofthefirstandsubsequentpierswithoutincludingatoolargecanyilevermomentduringconstruction.Thesecondandthefollowingsegmentsareconcreteddirectlyonthefaceofthehardenedportionandthelongitudinalreinforcementcancontinueacrosstheconstructionjoint.Thepushingisachievedbyhydraulicjackswhichactagainsttheabutment.SincethecoefficientoffrictionofTeflonslidingbearingsisonlyabout2percent,lowcapacityhydraulicjackswouldsufficetomovethebridgeevenoverlonglengthsofseveralhundredmetres.Thismethodcanbeusedforstraightandcontinuouslycurvedbridgesuptoaspanofabout120m.
Thefree-cantileversystemwaspioneeredbyDyckerhoffandWillmanningermany.Inthissystem,thesuperstructureiserectedbymeansofcantilevertruckinsectionsgenerallyof3.5m.Thecantilevertruck,whosecostisrelativelysmallandwhichisattachedfirmlytopermanentconstruction,ermitsbyrepeatedusetheconstructionoflargebridges.Theavoidanceofscaffoldfrombelow,thespeedofworkandthesavinginlabourcostresultintheconstructionbeingveryeconomicdal.Thefree-cantileversystemisideallysuitedforlaunchedgirderswithalargedepthabovethepiercantileversystemisideallysuitedforlaunchedgirderswithalargedepthabovethepiercantileveringtothemiddleofthespan.
Anothertechniqueistheuseofthepneumaticcaisson.Thecaissonisahugecylinderwithabottomedgethatcancutintothewaterbed.Whencompressedarispumpedintoit,thewaterisforcedout.Caissonsmustbeusedwithextremecare.foronething,workerscanonlystayinthecompressionchamberforshortperiodsoftime.Foranother,iftheycomeuptonormalatmosphericpressuretoorapidly,theyaresubjecttothebends,orcaissondiseaseasitisalsocalled,whichisacripplingorevenfatalconditioncausedbyexcessnitrogenintheblood.WhentheEadsBridgeacrosstheMississippiRiveratSt.Louiswasunderconstructionbetween1867and1874,atatimewhenthedangerofworkingincompresedairwasnotfullyunderstood,fourteendeathswascausedbythebends.
Whenextrastrengthisnecessaryinthepiers,theysometimeskeyedintothebedrock-thatis,theyareextendeddownintothebedrock.ThismethodwasusedtobuildthepiersfortheGoldenGateBridgeinSanFrancisco,whichissubjecttostrongtidiesandhighwinds,andislocatedinanearthquakezone.Thedrillingwascarriedoutunderwaterbydeep-seadivers.
Wherebedrockcannotbereached,pilesaredrivenintothewaterbed.Today,thepilesinconstructionareusuallymadeofprestressedconcretebeams.Oneingenioustechnique,usedfortheTappanZeeBridgeacrosstheHudsonRiverinNewYork,istorestahollowconcreteboxontopofalayerofpiles.Whentheboxispumpeddry,itbecomesbuoyantenoughtosupportalargeproportionoftheweightofthebridge(seeFig.22.3).
Eachtypeofbridge,indeedeachindividualbridge,presentsspecialconstructionproblems.Withsometrussbridges,thespanisfloatedintopositionafterthepiershavebeenerectedandthenraisedintoplacebymeansofjacksorcranes.Archbridgescanbeconstructedoverafalsework,ortemporaryscaffolding.Thismethodisusuallyemployedwithreinforcedconcretearchbridges.Withsteelarches,however,atechniquehasbeendevelopedwherebythefinishedsectionsareheldinplacebywiresthatsupplyacantileversupport.Cranesmovealongthetopofthearchtoplacenewsectionsofsteelwhilethetensioninthecablesincreases.
Withsuspensionbridges,thefoundionsandthetowersarebuiltfirst.Thenacableisrunfromtheanchorage-aconcreteblockinwhichthecableisfastened-uptothetowerandacrosstotheoppositetowerandanchorage.Awheelthatunwindswirefromareelqunsalongthiscable.Whenthereelreachestheotherside,anotherwireisplacedonit,andthewheelreturnstoitsoriginalposition.Whenallthewireshavebeenputinplace,anothermachinemovesalongthecabletocampactandtobindthem.Constructionbeginsonthedeckwhenthecablesareinplace,withworkprogressingtow
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