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
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