香港混凝土结构设计规范8最新版图文精.docx

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香港混凝土结构设计规范8最新版图文精

9DETAILINGOFMEMBERSANDPARTICULARRULES

9.1

GENERAL

Thedetailingrequirementsforsafety,serviceabilityanddurabilityaresatisfiedbyfollowingtherulesgiveninthissectioninadditiontothegeneralrulesgiveninsection8.

Thedetailingofmembersshouldbeconsistentwiththedesignmodelsadopted.

Minimumareasofreinforcementaregiveninordertopreventabrittlefailure,widecracksandalsotoresistforcesarisingfromrestrainedactions.

9.2BEAMS

9.2.1Longitudinalreinforcement

9.2.1.1Minimumpercentagesofreinforcement

Theminimumpercentagesofreinforcementappropriateforvariousconditionsofloadingaregivenintable9.1.

Minimumpercentage

SituationDefinitionof

percentage

fy=250

N/mm²

（%

fy=460

N/mm²

（%

Tensionreinforcement

Sectionssubjectedmainlytopuretension

Sectionssubjectedtoflexure:

100As/Ac0.80.45（iflangedbeams,webintension:

bw/b<0.4bw/b≥0.4100As/bwh

100As/bwh

0.32

0.24

0.18

0.13

（iiflangedbeams,flangeintension:

T-beamL-beam100As/bwh

100As/bwh

0.48

0.36

0.26

0.20

（iiirectangularsection100As/Ac0.240.13Compressionreinforcement

（wheresuchreinforcementisrequiredforthe

ultimatelimitstate

Generalrule100Asc/Acc0.40.4

Simplifiedrulesforparticularcases:

（irectangularbeam100A

sc/Ac0.20.2

（iiflangedbeam

flangeincompressionwebincompression100Asc/bhf

100Asc/bwh

0.4

0.2

0.4

0.2

Transversereinforcementinflangesof

flangedbeams

（providedoverfulleffectiveflangewidthnear

topsurfacetoresisthorizontalshear

100Ast/hfl0.150.15

Notes:

1.Theminimumpercentagesofreinforcementshouldbeincreasedwherenecessarytomeetthe

ductilityrequirementsgiveninclause9.9.

Table9.1-Minimumpercentagesofreinforcement

Sectionscontaininglessreinforcementthanthatgivenbytable9.1shouldbeconsideredasunreinforced.

9.2.1.2Sidebarsforbeamsexceeding750mmoveralldepth.

Theminimumsizeoflongitudinalbarsinsidefacesofbeamstocontrolcrackingshouldbenotlessthan√（sbb/fywheresbisthebarspacingandbthebreadthofthesectionatthepointconsidered,or

500mmifbexceeds500mm.

Thespacingshouldnotexceed250mmandthebarsshouldbedistributedoveradistanceoftwo-thirdsofthebeam’soveralldepthmeasuredfromitstensionface.

9.2.1.3Maximumareasofreinforcement

Neithertheareaoftensionreinforcementnortheareaofcompressionreinforcementshouldexceed4%ofthegrosscross-sectionalareaoftheconcrete.

Atlaps,thesumofthereinforcementsizesinaparticularlayershouldnotexceed40%ofthebreadthofthesectionatthatlocation.

9.2.1.4Maximumdistancebetweenbarsintension

Theclearhorizontaldistancebetweenadjacentbars,orgroups,nearthetensionfaceofabeamshouldnotbegreaterthan:

clearspacing≤70000βb/fy≤300mm9.1

where:

βb

istheredistributionratio:

tionredistribubeforesectiontheatmomenttion

redistribuaftersectiontheatmomentfromtherespectivemaximummomentsdiagram.

Alternatively,theclearspacingmaybeassessedfromtherelationship:

clearspacing≤47000/fs≤300mm

9.2

where:

fsistheestimatedservicestressinthereinforcementandmaybeobtainedfrom:

b

prov

s,reqs,ys1

32β×=

AAff9.3

Thedistancebetweenthefaceofthebeamandthenearestlongitudinalbarintensionshouldnotbegreaterthanhalfthecleardistancedeterminedabove.

9.2.1.5Otherdetailingarrangements

Inmonolithicconstruction,evenwhensimplesupportshavebeenassumedindesign,thesectionatsupportsshouldbedesignedforabendingmomentarisingfrompartialfixityofatleast15%ofthemaximumbendinginthespan.

（Note,theminimumareaoflongitudinalreinforcementdefinedinclause9.2.1.1applies.

Atintermediatesupportsofcontinuousbeams,thetotalareaoftensionreinforcementAsofaflangedcross-sectionmaybespreadovertheeffectivewidthofflange（seeclause5.2.1.2（aprovidedthatatleast50%ofAsisplacedwithinthewebwidth（seefigure9.1

9.2.1.6Curtailmentoftensionreinforcement

Exceptatendsupports（seeclause9.2.1.7ineveryflexuralmembereverybarshouldextendbeyondthepointatwhichintheoryitisnolongerneeded,foradistanceatleastequaltothegreaterof:

•theeffectivedepthofthemember;or•

12timesthebardiameter.

Inadditionforabarinthetensionzone,oneofthefollowingdistancesforallarrangementsofdesignultimateloadshouldbeconsidered:

•ananchoragelengthappropriatetoitsdesignstrength（0.87fyfromthepointatwhichitisnolongerrequiredtoassistinresistingthebendingmoment;or

•tothepointwherethedesignshearcapacityofthesectionisgreaterthantwicethedesignshearforceatthatsection;or

•tothepointwhereotherbarscontinuingpastthatpointprovidedoublethearearequiredtoresistthedesignbendingmomentatthatsection.

Thepointatwhichabarisnolongerrequiredisthepointwherethedesignresistancemomentofthesection,consideringonlythecontinuingbars,isequaltothedesignmoment.

Ascurtailmentofsubstantialareasofreinforcementatasinglesectioncanleadtothedevelopmentoflargecracksatthatpoint,itisthereforeadvisabletostaggerthecurtailmentpointsinheavilyreinforcedmembers.

Figure9.1-Placingoftensionreinforcementinflangedcross-section

9.2.1.7Anchorageofbarsatasimply-supportedendofabeam

Atasimply-supportedendofamemberhalfthecalculatedmid-spanbottomreinforcementshouldbeanchoredbyoneofthefollowing:

•aneffectiveanchoragelengthequivalentto12timesthebardiameterbeyondthecentre-lineofthesupport.Nobendorhookshouldbeginbeforethecentreofthesupport;or•aneffectiveanchoragelengthequivalentto12timesthebardiameterplusd/2fromthefaceofthesupport,wheredistheeffectivedepthofmember.Nobendorhookshould

beginbefored/2fromthefaceofthesupport.

9.2.1.8Anchorageofbottomreinforcementatinternalsupports

Atanintermediatesupportofacontinuousmember30%ofthecalculatedmid-spanbottomreinforcementshouldbecontinuousthroughthesupport.

9.2.1.9Cantileverextensions

Whereacantileverformsanextensionbeyondtheendsupportofacontinuousbeamorslab,careshouldbetakentoensurethatthetopsteelintheadjacentspanextendsbeyondthepointofcontraflexure.

9.2.1.10Containmentofcompressionreinforcement

Therecommendationsgiveninclause9.5.2.2forcolumnswillapplyalsotobeams.

9.2.2Shearreinforcement

Theshearreinforcementmayconsistofacombinationof:

•linksenclosingthelongitudinaltensionreinforcementandthecompressionzone;

•bent-upbars;and

•cagesorladders（seefigure9.2whicharecastinwithoutenclosingthelongitudinal

reinforcementbutareproperlyanchoredinthecompressionandtensionzones.

Linksshouldbeeffectivelyanchoredinaccordancewithclause8.5.Alapjointonthelegnearthe

surfaceofthewebispermittedprovidedthatthelinkisnotrequiredtoresisttorsion.

Atleast50%ofthenecessaryshearreinforcementshouldbeintheformoflinks.

Theminimumareaofshearreinforcementforthewholelengthofabeamisgivenby:

Asv≥0.4bvsv/（0.87fyv9.4

Themaximumspacingofthelinksinthedirectionofthespanshouldnotexceed0.75d.Atright-

anglestothespan,thehorizontalspacingshouldbesuchthatnolongitudinaltensionbarismore

than150mmfromaverticalleg.

Thelongitudinalspacingofbent-upbars（seefigure6.2shouldnotexceedsb,max,asgivenby

equation9.5.

sb,max=0.6d（1+cotα9.5where:

αistheinclinationofthebent-upbarstothelongitudinalaxis.

Figure9.2-Examplesofshearreinforcement

9.2.3Torsionreinforcement

Torsionlinksshouldbeclosedandbeanchoredbymeansoflapsorhookedends,seefigure9.3,and

shouldformanangleof90°withtheaxisofthestructuralelement.

Figure9.3-Torsionlinkarrangements

Torsionreinforcementshouldcomplywiththeprovisionsofclauses6.3.5to6.3.9.

SLABS

9.3SOLID

Thissectionappliestoone-wayandtwo-waysolidslabs.

9.3.1Flexuralreinforcement

9.3.1.1General

（aMinimumreinforcement

Thefollowingminimumpercentagesoftotallongitudinalreinforcementshouldbeprovidedin

eachdirection:

•fy=250N/mm2:

0.24%ofconcretecross-sectionalarea;and

•fy=460N/mm2:

0.13%ofconcretecross-sectionalarea.

Generallysecondarytransversereinforcementofnotlessthan20%oftheprincipal

reinforcementshouldbeprovidedinone-wayslabs.Inareasnearsupportstransverse

reinforcementtoprincipaltopbarsisnotnecessarywherethereisnotransversebending

moment.

（bReinforcementspacing

Themaximumspacingofbarsshouldcomplywiththefollowingrequirements:

•fortheprincipalreinforcement,3h≤400mm;and

•forthesecondaryreinforcement,3.5h≤450mm.

Inareaswithconcentratedloadsorareasofmaximummomentthoseprovisionsbecome

respectively:

•fortheprincipalreinforcement,2h≤250mm;and

•forthesecondaryreinforcement,3h≤400mm.

Inaddition,unlesscrackwidthsarecheckedbydirectcalculation,thefollowingruleswill

ensureadequatecontrolofcrackingforslabssubjectedtonormalinternalandexternal

environments:

Nofurthercheckisrequiredonbarspacingifeither:

•h≤250mm（grade250steel;

•h≤200mm（grade460steel;or

•thepercentageofrequiredtensionreinforcement（100As/bdislessthan0.3%.

Wherenoneofthesethreeconditionsapply,thebarspacingsshouldbelimitedtothevalues

calculatedinclause9.2.1.4forslabswheretherequiredreinforcementpercentageexceeds

1%orthevaluescalculatedinclaus