土木工程專業(yè)英語 課件 4.3. Bond,Anchorage,and Development Length

4.3Bond,Anchorage,andDevelopmentLength

粘結(jié)、錨固、基本錨固長度無粘結(jié)梁BeamwithunbondedinterfacebetweenreinforcementandconcreteIfthereinforcedconcretebeamofFigure4.17(a)wasconstructedusingroundreinforcingbars,and,furthermore,ifthosebarsweregreasedorlubricatedbeforetheconcretewerecast,thebeamwouldbeonlylittlestrongerthanifitwerebuiltofplainconcrete,withoutreinforcement.4.1FundamentalsofFlexuralBondIfaloadwereapplied,asshowninFigure4.17(b),thebarswouldtendtomaintaintheiroriginallengthasthebeamdeflects.Thebarswouldsliplongitudinallywithrespecttotheadjacentconcrete,whichwouldexperiencetensilestrainduetoflexure.Forreinforcedconcrete,itisessentialthatbondforcesbedevelopedonthe

interface

界面betweenconcreteandsteel,suchastopreventsignificantslipfromoccurringatthatinterface.Figure4.17(c)showsthebondforcesthatactontheconcreteattheinterfaceasaresultofbending,whileFigure4.17(d)showstheequalandoppositebondforcesactingonthereinforcement.ItisthroughtheactionoftheseinterfacebondforcesthattheslipindicatedinFigure4.17(b)isprevented.粘結(jié)應(yīng)力bondstress鋼筋與混凝土間具有足夠的粘結(jié)是保證鋼筋與混凝土共同受力變形的基本前提Someyearsago,whenplainbarswithoutsurfacedeformationswereused,initialbondstrengthwasprovidedonlybytherelativelyweakchemicaladhesionandmechanicalfrictionbetweensteelandconcrete粘附力摩擦力有滑移時粘附力即消失鋼筋受力較大時粘結(jié)力主要由此二部分組成chemicaladhesionfrictionOnceadhesionandstaticfrictionwereovercomeatlargerloads,smallamountofslipledtointerlockingofthenaturalroughnessofthebarwiththeconcrete.However,thenaturalbondstrengthissolowthatinbeamsreinforcedwithplainbars,thebondbetweensteelandconcretewasfrequentlybroken.

粘附力摩擦力有滑移時粘附力即消失鋼筋受力較大時粘結(jié)力主要由此二部分組成chemicaladhesionfriction機(jī)械咬合力（鋼筋表面不平、微銹時可顯著提高咬合力）interlockingnaturalroughness端布有錨固的無粘結(jié)梁BeamwithunbondedinterfacebetweenreinforcementwithendanchorageandconcreteTopreventthis,endanchoragewasprovided,chieflyintheformofhooks.Iftheanchorageisadequate,suchabeamwillnotcollapse,evenifthebondisbrokenovertheentirelengthbetweenanchorages.endanchorageThisissobecausethememberactsasatiedarch,asshowninFigure4.18,withtheuncrackedconcreteshownshadedrepresentingthearchandtheanchoredbarsrepresentingthetiedrod.

ZInthiscase,overthelengthinwhichthebondisbroken,bondforcesarezero.Thismeansthatovertheentireunbondedlengththeforceinthesteelisconstantandequaltoss=常數(shù)bondforcesarezero---unbondedbondstressforceinthesteelisconstantCzTMmaxAsaconsequence,thetotalsteelelongationinsuchabeamislargerthaninbeamsinwhichbondispreserved,resultinginlargerdeflectionsandgreatercrackwidths.CzTsteelelongation=T/(AsEs)TCT2<TbondispreservedToimprovethissituation,deformedbarsarenowuniversallyused.Withsuchbars,theshouldersoftheprojecting([prEJ`dVektIN]伸出的)ribs

bearonthesurroundingconcreteandresultingreatlyincreased[In`kri:st]strength.Itisthenpossibleinmostcasestodispensewith(廢除)specialanchoragedevicessuchashooks.Inaddition,crackwidthsaswellasdeflectionsarereduced.shoulders帶肋鋼筋deformedbarsribs4.2BondStrengthandDevelopmentLengthBondfailuresdirectpulloutofbarwhen鋼筋拔出

relativelysmalldiameterbars縱筋直徑小

sufficientlylargeconcretecoverthickness混凝土的保護(hù)層較大

sufficientbarspacing縱筋間距大

splitting

ofconcretealongbarwhen

順鋼筋方向混凝土劈裂insufficientcoverconfinement混凝土的保護(hù)層較小

insufficientbarspacing縱筋間距小Splitting

ofconcretealongbar順鋼筋方向劈裂裂縫ThesecondtypeoffailureissplittingoftheconcretealongthebarwhencoverconfinementorbarspacingisinsufficienttoresistthelateralconcretetensionresultingfromthewedgingeffectofthebardeformationsMechanicalInterlock.Theedgestressconcentrationcausescrackingtooccur.

WedgingeffectWedgingeffectlateralconcretetensionresultingfromthewedgingeffect(楔入效應(yīng))ofthebardeformations(ribs)◆變形鋼筋受力后，其凸出的肋對混凝土產(chǎn)生斜向擠壓力(bearingpressure)◆其水平分力(horizontalprojection)使鋼筋周圍的混凝土軸向受拉、受剪，徑向分力(radialforce)使混凝土產(chǎn)生環(huán)向拉力(hooptension)。◆軸向拉力和剪力使混凝土產(chǎn)生內(nèi)部斜向錐形裂縫◆環(huán)向拉力使混凝土產(chǎn)生內(nèi)部徑向裂縫。lateralconcretetension=axialforce+shearforce+hooptensionbearingpressure1.BondStrength變形鋼筋粘附力摩擦力機(jī)械咬合力主要作用chemicaladhesionfrictionMechanicalInterlockfrom

bardeformations

Formoderndeformedbars,adhesionandfrictionaremuchlessimportantthanthemechanicalinterlockofthedeformationswithsurroundingconcrete.Directpulloutofbars如果縱筋間距大且混凝土的保護(hù)層較大，徑向裂縫很難發(fā)展達(dá)到構(gòu)件表面，則肋前部的混凝土在水平分力和剪力作用下最終將被擠碎，發(fā)生沿肋外徑圓柱面的剪切破壞，形成所謂的“刮梨式”破壞，是變形鋼筋與混凝土粘結(jié)強(qiáng)度的上限。刮出式破壞Pull-outIfthebarissufficientlyconfinedbyamassofsurroundingconcrete,then,asthetensileforceonthebarisincreased,adhesive([Ed5hi:siv]

粘的)bondandfrictionareovercome,theconcreteeventuallycrusheslocallyaheadoftheribsimmediatelyadjacenttothebarinterface,andbarpulloutresults.Thesurroundingconcreteremainsintact.surroundingconcreteremainsintact變形鋼筋縱向分量徑向分量構(gòu)件縱向開裂Splitting

ofconcretealongbar順鋼筋方向劈裂裂縫Bondfailureresultingfromsplittingoftheconcreteismorecommoninbeamsthandirectpullout.Suchsplittingcomesmainlyfromwedgingactionwhentheribsofthedeformedbarsbearagainsttheconcrete.當(dāng)混凝土保護(hù)層和鋼筋間距較小時，徑向裂縫可發(fā)展達(dá)到構(gòu)件表面，產(chǎn)生劈裂裂縫，機(jī)械咬合作用將很快喪失，產(chǎn)生“劈裂式”粘結(jié)破壞lateralconcretetensionSplittingfailuremayoccureitherinaverticalplaneasinFigure4.20(a)orhorizontallyintheplaneofthebarsasinFigure4.20(b).Thehorizontaltypeofsplittingfrequentlybeginsatadiagonalcrack.Thedowelactionincreasesthetendencytowardsplitting.Thisindicatesthatshearandbondfailureareoftenintricatelyinterrelated.SplittingfailureinhorizontalplaneSplittingfailureinverticalplanediagonalcrackComplete[kEm5pli:t]failure完全失效occurswhenpulloutresistanceisovercomesplittinghasspreadallthewaytotheendofanunanchoredbar,Slidingofthesteelrelativetotheconcreteleadstoimmediatecollapseofthebeam.DevelopmentLengthLd

基本錨固長度Thedevelopmentlengthisdefinedasthatlengthofembedmentnecessarytodevelopthefulltensile

strengthofthebar,controlledbyeitherpulloutorsplitting.Thedevelopmentlengthistheshortest

lengthofbarinwhichthebarstresscanincreasefromzerototheyieldstrength鋼筋屈服時正好發(fā)生粘結(jié)破壞（鋼筋拔出或混凝土劈裂）原則

Thedevelopmentlengthofthebarisestablishedbypullouttests

la拔出試驗(yàn)ss=？L=Ld?bondforcesWhenreferenceto參考Figure4.21,themoment,andthereforethesteelstress,ismaximumatpointa(neglectingtheweightofthebeam)andzeroatthesupports.IfthebarstressisssAsata,thenthetotaltensionforcemustbetransferredfromthebartotheconcreteinthedistancelbybondforces.

Tofullydevelopthestrengthofthebar,thedistance

lmustbeatleastequaltothedevelopmentlengthofthebar,establishedbypull-outtests.la拔出試驗(yàn)ss=？L=Ld?bondforces

InthebeamofFigure4.21,iftheactuallengthlisequaltoorgreaterthanthedevelopmentlengthld,noprematurebondfailurewilloccur.Whenl≥ld,thebeamwillfailinbending,orfailinshear

NobondfailureThiswillbesoevenifinthevicinityofcrackslocalslipmayhaveoccurredoversmallregionalongthebeam.MainrequirementforsafetyagainstbondfailurelengthofthebarfromanypointofgivensteelstressssAs(oratmostfyAs)toitsnearbyfreeendmustbeatleastequaltoitsdevelopmentlength：l≥ldSpecialanchorage(hooks…)iftheactualavailablelengthisinadequateforfulldevelopment3FactorsInfluencingDevelopmentLengthExperimentalresearchhasidentifiedthefactorsthatinfluencedevelopmentlength,andanalysisofthetestdatahasresultedintheexperimentalequationsusedinpresentdesignpractice.Factorsinfluencingdevelopmentlengthconcretetensilestrength混凝土強(qiáng)度

coverthickness保護(hù)層厚度

spacingofthereinforcingbars鋼筋凈間距

transverse[5trAnzvE:s]steelreinforcement橫向配筋3FactorsInfluencingDevelopmentLengthconcretetensilestrength混凝土強(qiáng)度

ThetensilestrengthoftheconcreteisimportantbecausethemostcommontypeofbondfailureinbeamsisthetypeofsplittingThedevelopmentlengthisinverseproportionaltotheaxialtensilestrengthofconcrete,ft,butisnotinverseproportionaltothecubecompressivestrengthofconcrete,fcu.3FactorsInfluencingDevelopmentLengthcoverthickness保護(hù)層厚度

ifthehorizontalorverticalcoverisincreased,moreconcreteisavailabletoresistthetensionresultingfromthewedgingeffectofthedeformedbars,resistancetosplittingisimproved,anddevelopmentlengthisless.3FactorsInfluencingDevelopmentLength

barspacingifthebarspacingisincreased(e.g.,ifonlytwoinsteadofthreebarsareused),moreconcreteperbarwouldbeavailabletoresisthorizontalsplitting.Inbeams,barsaretypically[`tIpIkElI]spacedaboutoneortwobardiametersapart.Ontheotherhand,forslabs,footings(基礎(chǔ)）,andcertaintypesofmembers,barspacingsaretypicallymuchgreater,andtherequireddevelopmentlengthisreduced.3FactorsInfluencingDevelopmentLengthtransversesteelreinforcement橫向配筋

Transversereinforcementimprovestheresistanceoftensilebarstobothverticalfailureorhorizontalsplitting

failurebecausethetensionforceinthetransversesteeltendstopreventopeningoftheactualorpotentialcrack.DevelopmentLengthBasedontheresultofastatistical[stE5tistikEl]統(tǒng)計(jì)的analysisofpull-outtestsdata,withappropriate[E5prEupriit]simplifications,thelengthneededtodevelopstressfy

inareinforcingbarmaybeexpressedasapparentcoefficientofsteelbar.bardiameterAnchorageofTensionBarsbyHooksStandardDimensionsIntheeventthatthedesiredtensilestressinabarcannotbedevelopedbybondalone,itisnecessarytoprovidespecialanchorageattheendofthebar,usuallybymeansofa90degreesora180degreeshook.Thedimensionsandthebendradii[5reidiai]forsuchhookshavebeenstandardized[5stAndE7daizd]使標(biāo)準(zhǔn)化incoderadii--Aplural[5pluErEl]ofradiusA180degreesbendplusanextensionofatleast3bardiametersatthefreeendofthebar(2)A90degreesbendplusanextensionofatleast15bardiametersatthefreeendofthebarForstirrupandtieanchorage,a135degreesbendplusanextensionofatleast5bardiameters,butnotlessthan50mm,ora90degreesbendplusanextensionofatleast10bardiametersatthefreeendofthebarMechanicalAnchorage機(jī)械錨固Forsomespecialcases,e.g.（exempligratia[i^5zemplai5^reiFiE]例如=forexample）,attheendsofmainflexuralreinforcementindeepbeams,thereisnoroomforhooksorthenecessary[5nesEsErI;(?@)5nesEserI]confinementsteel,andspecialmechanicalanchoragedevicesmustbeused.Theymayconsistofweldedshorttransversebar,weldedplate,orT-headedbar.weldedshorttransversebarweldedplateT-headedbarTheconnectionofsteelbarscanbedividedintotwotypes:colligationandmechanicalconnectionorwelding.Therearethreechoicesforjoiningbarstogether(1)mechanicaldevice,(2)welding,and(3)lapsplices.Weldingjointshouldsatisfythestipulationsinrelatedexistingstandardsofthestate.Weldedsplicesmustconformtostructuralweldingcodeforreinforcingsteel.Whichisbetter?4.3.4鋼筋的連接BarSplices搭接長度翻譯：受力鋼筋的接頭宜設(shè)置在受力較小處。

Itisadvisabletosetthejointofareinforcingbarwheretheborneforceisrelativelysmaller.

Splicesinthereinforcementatpointsofmaximumstressshouldbeavoided.

Whichisbetter?4.3.4鋼筋的連接BarSplices綁扎搭接lapsplices

機(jī)械連接mechanicaldevice鋼筋連接形式焊接weldingThreechoicesforjoiningbarstogether:lapsplicesThemechanicalandweldedsplicesmustbetestedtoshowthedevelopmentintensionorcompressionofatleast125%ofthespecified[5spesifai]yieldstrength

ofthebar.Weldedsplicesmustconformtostructuralweldingcodeforreinforcingsteel.Sincesplicesintroduceweaklinksintothestructure,theyshouldbelocatedasmuchaspossibleawayfrompointsofmaximumforceandcriticallocations.Specify--Tostateexplicitlyorindetail:明確說明適用范圍：■Splicesforbarssmallerthan16mmindiameterareusuallymadesimplybylappingthebarsasufficientdistancetotransferstressbybondingfromonebartotheother.

通過鋼筋與混凝土之間的粘結(jié)強(qiáng)度來傳遞鋼筋的內(nèi)力。■

Thelappedbarsareusuallyplacedincontactandtightlywiredsothattheystayinpositionastheconcreteisplaced.lapsplicessufficientdistancetightlywiredplacedincontact適用范圍：

■Designcodeprohibits[prE5hibit]theuseoflappedsplicesformainreinforcementinaxiallyloadedtensionmembersandtensionmembers