Structural Design - Post Tensioned Slab Design1

8/17/2019 Structural Design - Post Tensioned Slab Design1

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Prepared by: Eng. Chamil Duminda

Mahagamage B.Sc.Eng (Hons), C Eng,1

Design of Pre-Stressed Flat Slabs

Code o" Prac#ice $ EC% Design o" concre#e s#ruc#ures

Introduction The technique of prestressing has several dierent applications withincivil engineering , often being used to eep cables taut when sub!ectedto co"pressive forces# $owever, b% far the "ost co""on applicationis in prestressed concrete where a prestress force is applied to a

concrete "e"ber, and this induces an a&ial co"pression thatcounteracts all, or part of, the tensile stresses set up in the "e"ber b%applied loading#'ithin the (eld of building structures, "ost prestressed concreteapplications are in the for" of si"pl% supported precast oor and roofbea"s# These are usuall% factor% "ade, where the advantages ofcontrolled "ass production can be reali*ed# 'here large spans arerequired, in situ prestressed concrete bea"s are so"eti"es used, andin situ prestressed concrete at slab construction is increasingl% beinge"plo%ed#+ost prestressed slabs utili*e unifor" depth slabs with draped

tendons, but an interesting develop"ent is the use of variable-depthslab with straight tendons#


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Prepared by: Eng. Chamil Duminda Mahagamage B.Sc.Eng (Hons), C Eng,ME(S!)

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Two way load balancing one wa% spanning slab is essentiall% a ver% wide si"pl% supported

bea"# 'ith a prestressed concrete slab si"pl% supported on fouredges, the situation is dierent, since the structure is now highl%indeter"inate# rectangular slab is shown in 8igure below supportedon walls along each edge and prestressed with sets of unifor"l%spaced parabolic tendons in each direction#


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9f the prestress forces in the & and % directions are, respectivel% :& and:% per unit length, and the "id span eccentricities are e& and e%,respectivel%, then since the vertical forces due to the tendons in eachdirection are additive at an% point in the slab, the total upward unifor"

load on the slab ;;##

ince the tendons "ust have a "ini"u" spacing between the", thestress distribution within the slab will not be e&actl% unifor", but inpractice it would be reasonabl% so# :restressed concrete slabs such asthat shown in (gure above are rarel% found in practice, and the "oreco""on for" is the at slab#


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The slab shown above will resist an% additional i"posed load in "uchthe sa"e wa% as would a reinforced concrete at slab, and the sa"eanal%sis "ethods are applicable# ince this anal%sis need onl% becarried out for a s"all percentage, usuall%, of the total design load,

an% inherent inaccuracies in the "ethod are not signi(cant#


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9n practice, the sharp changes of curvature in the tendons are avoidedb% adopting s"ooth reversing in curvature# n alternative pattern oftendons which is often used based on the distribution of pri"ar% andsecondar% tendons is well suited to slabs with irregular colu"n

la%outs, but shear strength will be reduced#


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Equivalent Frame Analysis The load balancing technique is ver% useful for esti"ating the prstressforce required in each direction, but an anal%sis of the slab for the

unbalanced loads "ust still be "ade# The anal%sis and design ofreinforced concrete at slabs with regularl% spaced colu"ns has beenbased on a "ethod which divides the slab and colu"ns into a series ofequivalent fra"es in each direction# The distributions of bending"o"ent and shear force "a% be deter"ined b% an% of the available"ethods of structural anal%sis# The equivalent fra"e "ethod has also

been found to give acceptable results for prestressed concrete atslabs and is one of those reco""ended in T54# The equivalent fra"e "ethod can be used to (nd the distribution of"o"ents due to both the prestress force and the total design load,and not !ust the unbalanced loads# ach equivalent fra"e co"prisescolu"ns and strips of slab at each oor level# The width of slab to be

used to deter"ine the bea" stiness is generall% equal to the fullwidth of the panels for vertical loading, whilst for hori*ontal loading,where lateral stabilit% is provided b% the fra"e, one-half of the panelwidth should be used#9f the equivalent fra"e "ethod is used to deter"ine the stressesunder the design load, the anal%sis should be carried out for the

following load cases#i ll s ans loaded with total desi n load


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0onsideration should be given to an% restraint to the free "ove"ent of

a prestressed concrete slab due to a&ial shortening, and creep andshrinage eects# This restraint will have the eect of reducing theprestress in the slab and is caused b% the colu"ns and an% wallsconnected to the slab such as those fro" lift shafts, for instance# s aguide if the a&ial stress in the slab is less than 2./""2, and there isnot "ore than one sti restraint between "ove"ent !oints, then

restraint eect is usuall% ignored#


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Design and Detailing8or preli"inar% esti"ation of the required slab depth, span/depthratios in the range of 6-4< for oors and roofs, respectivel%, "a% be

assu"ed# 'a>e slabs are so"eti"es used, and the span/depth ratiofor these should be taen as 2


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Antensioned reinforce"ent should be placed in the top of all slabs overthe colu"ns, equal in area to a "ini"u" of ?#?7


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Structural Design - Post Tensioned Slab Design1

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