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1、20*TimberBridges20.1IntroductionTimberasaBridgeMaterial•Past,Present,andFutureofTimberBridges20.2PropertiesofWoodandWoodProductsPhysicalPropertiesofWood•MechanicalPropertiesofWood•WoodandWood-BasedMaterialsforBridgeConstruction•PreservationandProtection20.3Typeso
2、fTimberBridgesSuperstructures•TimberDecks•Substructures20.4BasicDesignConceptsSpecificationsandStandards•DesignValues•KennethJ.FridleyAdjustmentofDesignValues•BeamDesign•WashingtonStateUniversityAxiallyLoadedMembers•Connections20.1IntroductionWoodisoneoftheearlies
3、tbuildingmaterials,andassuchoftenitsusehasbeenbasedmoreontraditionthanonprinciplesofengineering.However,thestructuraluseofwoodandwood-basedmaterialshasincreasedsteadilyinrecenttimes,includingarenewedinterestintheuseoftimberasabridgematerial.Supportingthisrenewedi
4、nteresthasbeenanevolutionofourunderstandingofwoodasastructuralmaterialandourabilitytoanalyzeanddesignsafe,durable,andfunctionaltimberbridgestructures.Anaccurateandcompleteunderstandingofanymaterialiskeytoitsproperuseinstructuralapplications,andstructuraltimberand
5、otherwood-basedmaterialsarenoexceptiontothisrequire-ment.Thischapterfocusesonintroducingthefundamentalmechanicalandphysicalpropertiesofwoodthatgovernitsstructuraluseinbridges.Followingthisintroductionofbasicmaterialprop-erties,apresentationofcommontimberbridgetyp
6、eswillbemade,alongwithadiscussionoffundamentalconsiderationsforthedesignoftimberbridges.20.1.1TimberasaBridgeMaterialWoodhasbeenwidelyusedforshort-andmedium-spanbridges.Althoughwoodhasthereputationofbeingamaterialthatprovidesonlylimitedservicelife,woodcanprovidel
7、ong-standingandserviceablebridgestructureswhenproperlyprotectedfrommoisture.Forexample,manycoveredbridgesfromtheearly19thcenturystillexistandareinuse.Today,ratherthanprotectingwood*PartsofthischapterwerepreviouslypublishedbyCRCPressinHandbookofStructuralEngineeri
8、ng,W.F.Chen,Ed.,1997.©2000byCRCPressLLCbyaprotectiveshelteraswiththecoveredbridgeofyesteryear,woodpreservativeswhichinhibitmoistureandbiologicalattackhavebeenu
