4.1.FlexuralBehaviorofReinforceConcreteBeam(2)

TbeamsWiththeexceptionofprecastsystem,reinforcedconcretefloor,roofs,decks,etc.,arealmostalwaysmonolithic.precastsystemmonolithicTbeamsFormsarebuiltforbeamsoffitsidesandfortheundersideofslab,andtheentireconstructioniscastatonce,fromthebottomofthedeepestbeamtothetopoftheslab.Soffit--Theundersideofastructuralcomponent,suchasabeam,arch,staircase,Tbeams--FlangedSectionSlabactsasatopflangetothebeam;T-beams,andInvertedL(Spandrel)Beams.PositiveandNegativeMomentRegionsinaT-beam

ReinforcedconcretefloorsystemTbeams--FlangedSectionThesectionA-Aatmidspanissubjectedtoapositivemoment,apartoftheslabwillactwiththeupperpartofthebeamtoresistlongitudinalcompression.ThesectionB-Batthesupportissubjectedtoanegativemoment,theslabbeingundertensionandignored,andthebeamwidthisthatoftheweb.TheresultingbeamcrosssectionisT-shapedratherthanrectangular.Theslabformsthebeamflange,whilethepartofthebeamprojectingbelowtheslabformswhatiscalledtheweborstem.EffectiveFlangeWidthPortionsnearthewebsaremorehighlystressedthanareasawayfromtheweb.EffectiveFlangeWidththecompressivestresses,inaT-section,areatamaximumvalueatpointsadjacenttothebeamanddecreaseapproximatelyinaparabolicformtozeroatadistancefromthefaceofthebeam.Stressesalsovaryverticallyfromamaximumatthetopfibersoftheflangetoaminimumatthelowerfibersoftheflange.compressivestressesvaryhorizontallycompressivestressesvaryhorizontallyEffectiveFlangeWidthAsameansofsimplification,ratherthanvaryingwithdistancefromtheweb,aneffectiveflangewidth

ofuniformstressmaybeassumed.Theeffectivewidthdependprimarilyonthebeamspanandontherelativethicknessoftheslab.StrengthAnalysisTheneutralaxisofaTbeammaybeeitherintheflange(TypeITsection)orintheweb(TypeIITsection),dependingupontheproportionofthecrosssection,theamountoftensilesteel,andthestrengthofmaterial.Ifthecalculateddepthtotheneutralaxisislessthanorequaltotheslabthickness,thebeamcanbeanalyzedasifitwerearectangularbeamofwidthequaltotheeffectiveflangewidth,asshowninFigure4.8.Whentheneutralaxisisintheweb,asinFigure4.9,aTbeamanalysisisrequiredasfollows.TypeITsection--calculateddepthtotheneutralaxisislessthanorequaltotheslabthickness.TypeIITsection–neutralaxisisintheweb第一類T形截面的計(jì)算公式與寬度等于bf’的矩形截面相同

為防止超筋脆性破壞，相對(duì)受壓區(qū)高度應(yīng)滿足x≤xb。對(duì)第一類T形截面，該適用條件一般能滿足；為防止少筋脆性破壞，受拉鋼筋面積應(yīng)滿足As≥rminbh，b為T(mén)形截面的腹板寬度；對(duì)工形和倒T形截面，則受拉鋼筋應(yīng)滿足As≥rmin[bh+(bf

-

b)hf]

BeamcanbeanalyzedasifitwerearectangularbeamofwidthequaltotheeffectiveflangewidthBeamAnalysisofTypeIITSectionAsacomputationaldevices,itisconvenienttodividethetotalreinforcementintotwopartsThefirstpartAs1representsthesteelareawhich,whenstressedtofy,isrequiredtobalancethelongitudinalcompressiveforceintherectangularportionofthebeam.TheremainingpartAs2,atastressfy,isbalancedbythelongitudinalforceintheoverhangingportionoftheflangethatarestresseduniformlyatoverhangingConstructionalDetailsofBeams

截面尺寸和配筋構(gòu)造ConcreteProtectionforReinforcement混凝土保護(hù)層厚度Toprovidethesteelwithadequateconcreteprotectionagainstfireandcorrosion,acertainminimumthicknessofconcretecoveroutsideoftheoutermostsurfaceofsteelmustbemaintainedInadequateconcretecoveroverthesteelreinforcementhascausedthesteeltocorrode.Asitexpandsitcausestheconcretetocrackandspall.混凝土保護(hù)層ConcreteCoveringMinimumthicknessofconcretecoverrequiredwillvary,dependingonthetypeofmemberconditionofexposure.Forcast-in-placeconcrete,concreteprotectionatsurfacenotexposeddirectlytothegroundorweathershouldnotbelessthan15mmforslabsandwalls,and25mmforbeamsandcolumns.Iftheconcretesurfaceistobeexposedtotheweatherorincontactwiththeground,aprotectingcoveringofatleast20mmisrequiredforslabsandwalls,and30mmforbeamsandcolumns

一類環(huán)境中梁、板的混凝土保護(hù)層厚度一般取為：混凝土強(qiáng)度等級(jí)≤C20時(shí)，梁30mm，板20mm；混凝土強(qiáng)度等級(jí)≥C25時(shí)，梁25mm，板15mmMinimumthicknessofconcretecoverConcreteProportionsReinforcedconcretebeammaybewideandshallow,orrelativenarrowanddeep.Considerationofmaximummaterialeconomyoftenleadstoproportionswithbeamdepthhintherangefromabout2to3.5(

or

2.5to4forTbeam)timesthewidth

b.

梁適宜的截面高寬比h/b，矩形截面為2～3.5，T形截面為2.5～4。hbhbwideandshallownarrowanddeepConcreteProportionsTosimplifyconstructionandtherebytoreducecost,thebeamdepthhisalwaysroundedupwardtothenexthigher50mmwhenhislessthanorequalto800mm,andtothenexthigher100mmwhenhisgreaterthan800mm.

梁的截面高度h一般可取250、300…800、900、1000㎜等，h≤800mm時(shí)以50mm為模數(shù)，h＞800mm時(shí)以100mm為模數(shù)；Thebeamwidthb(orwebwidthforTbeam)shouldbe100,120,150,180,200,220,250mm,andshouldberoundedupwardtothenexthigher50mmwhenbisgreaterthan250mm.

矩形梁的截面寬度和T形截面的肋寬b宜采用100、120、150、180、200、220、250mm，大于250mm時(shí)以50mm為模數(shù)。SelectionofBarsandBarSpacingCommonreinforcementbarsizesrangesfrom12mmto25mm.梁縱向受力鋼筋的常用直徑d=12~25mm。

Itisnecessarytomaintainacertainminimumdistancebetweenadjacentbarstoensureproperplacementofconcrete鋼筋間距BarSpacingClearhorizontalspacingbetweenparalleltensionbarsSelectionofBarsandBarSpacingTheminimumclearhorizontalspacingbetweenparalleltensionbarsinalayershallnotbelessthan25mmor1bardiameter.Thecleardistancebetweenadjacentcompressionbarsshallnotbelessthan30mmor1.5bardiameter.凈距

25mm

鋼筋直徑dcccbhc25mm

dh0bhh0凈距

30mm1.5鋼筋直徑d凈距

25mm

鋼筋直徑dWhenparallelreinforcementisplacedintwoormorelayers,designcoderequiresthatlongitudinalbarsshallhaveaclearverticalspacingnotlessthan25mmor1.5bardiameter,andthebarsintheupperlayershouldbeplaceddirectlyabovethoseinthebottomlayer.凈距

25mm

鋼筋直徑dcccbhc25mm

dh0bhh0凈距

30mm1.5鋼筋直徑d凈距

25mm

鋼筋直