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Post-tensioned Concrete Floors A GUIDE TO DESIGN AND CONSTRUCTION Post-tensioned Concrete Floors PAGE ii Contents 1 2 development of Post-tensioned Floors Principles of Post-tensioned Floors INTrodUCTIoN In the UK, the use of post-tensioned (PT) concrete floors in buildings is now commonplace. Post-tensioned floor slabs are also widely used in multistorey construction overseas, particularly in North America, Australia and the Middle East. In California it is the primary choice for concrete
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  Pst-tensineCncete Fls A GUIDE TO DESIGN AND CONSTRUCTION  Post-tensioned Concrete Floors PAGE ii In the UK, the use  pst-tensine (PT) cncete fs in builings isnw cmmnplace. Pst-tensine f slabs ae als wiely use in multi-stey cnstuctin veseas, paticulaly in Nth Ameica, Austalia anthe Mile East. In Calinia it is the pimay chice  cncete fs. INTrodUCTIoN Cve pictues:Main: Pst tensining at Paaise Steet, Livepl- a mixe use evelpment  etail an ca paking. Cutesy  Cnce. Inset: Bigewate Place, Lees.- a mixe use evelpment  32 steys. Cutesy  Bigewate Place Lt an Stuctual SystemsUK Lt. In the UK, typical applicatins have been: ãOfcesãApartmentbuildingsãCarparksãShoppingcentresãHospitalsãTransferbeams. The pupse  this publicatin is t wienthe unestaning  pst-tensine f constructionandshowtheconsiderablebenets an pptunities it es t evelpes, achitects, engineersandcontractors.Thesebenetsinclude:ãMinimumstoreyheightsãMinimumnumberofcolumnsãRapidconstructionãEconomyãMaximumdesignexibilityãOptimumclearspansãJoint-free,crack-freeconstructionãControlleddeections. This publicatin als aims t ispel the mythsabut pst-tensine cncete slabs an answesequently aske questins by shwing that: ãThedesignisnotnecessarilycomplicated.ãPToorsarecompatiblewithfast-track cnstuctin. ãPToorsdonotrequiretheuseofhigh-strength cncete. ãTheformworkdoesnotcarryanyofthe pestessing ces. ãPToorscanbedemolishedsafely.ãLocalfailuredoesnotleadtototalcollapse.ãHolescanbecutinslabsatalaterdate. A me etaile guie t the esign  PT fs canbe un in The Cncete Sciety Technical reptTr43 Post-tensioned Concrete Floors:Design Handbook [1].   Cainal Place, Lnn. Cutesy  Feyssinet. Cntents 1 develpment Pst-tensine Fls2 Pinciples Pst-tensine Fls 3Benetsof   Pst-tensine Fls6 Stuctual Fms 7 PT Flat Slab8 PT ribbe an Wafe Slabs9 PT Beam an Slab 10 design They12 design Cnsieatins15 Cnstuctin Cnsieatins16 Cst Cmpaisns   16 Cmmecial Builings 18Hospitals 19 Schls 20 En  Lie21 Summay21 reeences  Post-tensioned Concrete Floors PAGE 1 dEVELoPMENT oF PoST-TENSIoNEd FLoorS The pactice  pestessing can be tace back as aas 440BC, when the Geeks euce bening stesses andtensionsinthehullsoftheirghtinggalleysby pestessing them with tensine   the simplest examples  pestessing isthat  tying t lit a w  bks as illustate inFigue 1 belw. T lit the bks it is necessay tpush them tgethe, i.e. t apply a pecmpessint the w. This inceases the esistance t slipbetween the bks s that they can be lite.In the 19th centuy, seveal enginees tie tevelp pestessing techniques withut success. Theinventin  pestesse cncete is acceite t EugeneFreyssinetwhodevelopedtherstpractical pst-tensining system in 1939. Systems weeevelpe aun the use  multi-wie tennslcate in lage ucts cast int the cncete sectin, andxedateachendbyanchorages.Theywere stesse by jacking m eithe ne  bth ens,an then the tenns wee gute within the uct.This is geneally eee t as a bne system asthe guting bns the tenn alng the length the sectin.The bning is simila t the way in which bas aebne in eince cncete. Ate guting is cmpletethee is n lnge any eliance n the anchage ttanse the pecmpessin int the sectin.Applicatins in builings have always existe inthe esign  lage span beams suppting heavylaings, but these systems wee nt suitable pestessing f slabs, which cannt accmmateeithe the lage ucts  anchages.duing the 1960s, in the US, unbne systems weeevelpe. These ely n the anchages t tansethe ces between the stan an the cncetethughut the lie  the stuctue.Me vesatile bne systems suitable  fslabs wee evelpe in Austalia. Bne systemsbecame ppula in the UK in the 1990s. In the UK,bne cnstuctin is nw wiely use; havingappximately 90%  the PT suspene f maket.Bth bne an unbne systems ae suitable f slabs an a cmpaisn  the techniquesis given in the sectin n design Cnsieatins(page 14). Unbne PT tennBne PT cmpnents SheathStanGease The ‘pre’ in pre-stressing describes the stress applied before any normal loads are applied. The ‘post’ in post-tensioned refers to the strandsbeing tensioned after the concrete has been cast and gained sufcient strength to be compressed in an equal and opposite reaction to thetensioning of the strands. Figure 1: liFting a row oF books Unbne system bee puing cncete. Cutesy  Balvac. Bne system bee puing cncete. Cutesy  Feyssinet.  Post-tensioned Concrete Floors PAGE 2 Concrete has a low tensile strength but is strong in compression. By pre-compressing aconcrete element, so that when exing under applied loads it still remains in compression,a more efcient design for the structure can be achieved. The basic principles of prestressedconcrete are given in Figure 2. PrINCIPLES oFPoST-TENSIoNEd FLoorS Une an applie la, a pestesse element willben, eucing the built-in cmpessin stesses;when the la is emve, the pestessing cecauses the element t etun t its iginal cnitin,illustating the esilience  pestesse cncete.Futheme, tests have shwn that a vituallyunlimite numbe  such evesals  the laing canbe caie ut withut aecting the element’s abilityt cay its wking la  impaiing its ultimatela capacity. In the ws pestessing enws theelement with a high egee  esistance t atigue.I the tensile stesses ue t la  nt excee thepestess, the cncete will nt cack in the tensinzne. I the wking la is exceee an the tensilestesses vecme the pestess, cacks will appea.depening n the envinment it may be acceptable tohavesomecracking.However,evenafteran element has been lae t beyn its wking la,an well twas its ultimate capacity, emval  thela esults in clsing  the cacks an they will nteappea une wking la.Thee ae tw meths  applying pestess t acncete membe. These ae: ãPost-tensioning-wheretheconcreteisplaced  aun sheaths  ucts cntaining unstesse tendons.Oncetheconcretehasgainedsufcient  stength the tenns ae stesse against thecncete an lcke  by special anch gips,knwn as split weges. In this system, all tennces ae tansmitte iectly t the cncete.Since n stesses ae applie t the mwk,cnventinal mwk may be use. ãPre-tensioning-wheretheconcreteisplaced  aun peviusly stesse tenns. As thecncete haens, it gips the stesse tenns an whenithasobtainedsufcientstrengththetendons  ae elease, thus tanseing the ces t thecncete. Cnsieable ce is equie t stess thetenns, s pe-tensining is pincipally use pecast cncete whee the ces can be estaine byxedabutmentslocatedateachendofthe  stessing be,  caie by specially stiene muls. Prestressedconcretecanmosteasilybedenedasprecompressedconcrete. This means that a cmpessive stess is put int a cncete membe beeit begins its wking lie, an is psitine t be in aeas whee tensilestesses wul thewise evelp une wking la. Cnsie a beam plain cncete caying a la.Une la, the stesses in the beam will be cmpessive in the tp antensile in the bttm. We can expect the beam t cack at the bttm,even with a elatively small la, because  cncete’s lw tensile stength.Thee ae tw ways  cunteing this lw tensile stength - by using steeleincement  by pestessing.In pestesse cncete, cmpessive stesses ae intuce int aeaswhee tensile stesses will evelp une la t esist  annul thesetensile stesses. S the cncete nw behaves as i it ha a high tensilestength  its wn.In eince cncete, eincement in the m  steel bas is placein aeas whee tensile stesses will evelp une la. The eincementcaies all the tensin an, by limiting the stess in this eincement, thecacking  the cncete is kept within acceptable limits.(a)(b) Figure 2: principles oF prestressing (c)() TermDenition Anchorage device t lck the stan at ape-etemine tensile ce,which inuces cmpessive stessin the cncete. Dead-endanchorage An anchage whee n jackingtakes place. Duct Metal  plastic tube thughwhich the stan is passe  thebne system. Eccentricity distance between the centi the cncete sectin an thecente  the stan. Live anchorage The anchage at the jacking en the stan. Bth ens  thestan can be live. Prole Gemetic shape  the tenn inelevatin, ten paablic. Sheath Plastic extusin mule iectlyt the stan. A laye  geasebetween the stan an thesheath pevents bning. Strands Highstrengthsteelreinforcement. Tendon one  me stans in acmmn uct  sheath. Table 1: Post-tensioning terms
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