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A beam or "girder" bridge is the simplest and most inexpensive kind of bridge. According to Craig Finley of Finley/McNary Engineering, "they're basically ... BeamBridge Beambridge Abeamor"girder"bridgeisthesimplestandmostinexpensivekindofbridge. AccordingtoCraigFinleyofFinley/McNaryEngineering,"they'rebasicallythe vanillasofthebridgeworld." Initsmostbasicform,abeambridgeconsistsofahorizontalbeamthatis supportedateachendbypiers.Theweightofthebeampushesstraightdownon thepiers. Thebeamitselfmustbestrongsothatitdoesn'tbendunderitsownweightand theaddedweightofcrossingtraffic.Whenaloadpushesdownonthebeam,the beam'stopedgeispushedtogether(compression)whilethebottomedgeis stretched(tension). TryIt! Whathappenswhenaloadpushesdownonabeambridge? Takeaflateraserorasmallspongeandsliceashallownotchacrossthetop andbottom.Createabeambridgebysupportingeachendoftheeraser(or sponge)withastackofbooks.Pressdownonthecenterofthebridge.What happenstothetopandbottomnotches?Noticehowthetopnotchsqueezes togetherincompression,whilethebottomnotchspreadsapartundertension. Pre-stressedconcreteisanidealmaterialforbeambridgeconstruction;the concretewithstandstheforcesofcompressionwellandthesteelrodsimbedded withinresisttheforcesoftension.Pre-stressedconcretealsotendstobe oneoftheleastexpensivematerialsinconstruction.Buteventhebest materialscan'tcompensateforthebeambridge'sbiggestlimitation:its length. Thefartherapartitssupports,theweakerabeambridgegets.Asaresult, beambridgesrarelyspanmorethan250feet.Thisdoesn'tmeanbeambridges aren'tusedtocrossgreatdistances—itonlymeansthattheymustbe daisy-chainedtogether,creatingwhat'sknowninthebridgeworldasa "continuousspan." LakePonchartrainCauseway,Louisiana Infact,theworld'slongestbridgeisacontinuousspanbeambridge.Almost 24mileslong,theLakePonchartrainCausewayconsistsoftwo,two-lane sectionsthatrunparalleltooneanother.TheSouthboundLane,completedin 1956,ismadeupof2243separatespans,whiletheNorthboundLane,completed in1969,ispiecedtogetherfrom1500longerspans.Sevencross-overlanes connectthetwomainsectionsandfunctionaspull-overbaysinemergencies. Althoughimpressive,theLakePonchartrainCausewaybridgeunderscoresthe drawbackofcontinuousspans:theyarenotwellsuitedforlocationsthat requireunobstructedclearancebelow. (backtointro) (nextbridgedescription:suspensionbridge) Photos:(1)RitaSerderian-Precast/PrestressedConcreteInstitute;(2)GulfEngineersandConsultants,Inc. BuildABridge| Resources| Guide| Transcript| BridgeHome Editor'sPicks| PreviousSites| JoinUs/E-mail| TV/WebSchedule AboutNOVA| Teachers| SiteMap| Shop| Jobs| Search| Toprint PBSOnline| NOVAOnline| WGBH ©|UpdatedOctober2000

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