unbond pt slab economic alt

8/3/2019 Unbond PT Slab Economic Alt

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Unbonded-TSlabssAnEconomical telnatiueundreds of mil l ions ofsquare feet of post - ten-sioned (PT) concrete slabshave been built throughoutNorth America since the mid-

1960s.1n the basis f thisquantityof construction lonemany owners,contractors,and consultantswouldconsiderPT concrete labs o be aviablemethodof construction or avarietyof structures.Nevertheless,n someareas hereexistsa reluctanceo consider hi salternativesystemseriously.Thiscan usuallybe attributed o one ormore of the followingfactors:. Lack of familiarity with the sys-tem by ownersand contractorsr Lack of familiarity with analysisand designprocedures y con-sultantso Insufficientnumberof supplierso Concernabout the tensile orceslocked nto PT tendonso CostHowever, exper ience ver thepast25 yearshas proven hat fearsabout he useof PT slabsar e un -founded. More important ly, PTslabscarry with them nherentben-efits no t availablewith other sys-tems,and are a viablealternative oregular ly einforced(RR) slabs.

Combinedwith the fact that PTconcreteslabscan, in many in-stances, e lesscostly o constructthan their RR counterparts, henthe ownermay be sufferinga dis-service f PT slabsare excludedfrom consideration.ThesystemPost-tensionings a form of pres-tressingn which high-strength a-ble,which s usedas he primaryre-

inforcement, s tensioned fter theconcretes placed. n PT concreteslabs,one generallywants o pro-vide sufficient orces n the cablesoinhibit tensioncracks rom formingunderworking oads. n this way,the entire concretesection s en-gaged,allowing onger spanswithshal lower,and therefore ighter,sections. ince he grosssection sactive,deflectionsunder workingloads are either reducedor elimi-nated.PT concrete onstruction s clas-sif ied as either bonded or un-bonded:The high-strength trandsareeitherbonded o the concrete ygrouting the tendon sheathafterstressing, r they are greased ndplasticcovered, hus remainingun-bonded rom the concrete. he sYs-tem uses ither0.5 or 0.6 in. (12.7or 15.2mm) seven-wire trands5with a guaranteed ltimate ensilestrength GUTS) of 41.3 or 58.6kips(184or 261kN), respectively.Typical ly,bondedPT is mult i-st rand, with 2 to more than 50strandsenclosed n either flexiblemetalor rigid thin-wallsheaths ithdiametersanging rom 1.5 to over7 in. (40 o 180mm). I ts appl ica-tions ncludebridges, ircular iquidstorage anks, and liningsof pres-sure unnels.Their diameter, swellas he time and cost associated ithgrouting usually all but precludestheir use n slabs.UnbondedPT is a single-strandsystem.The indivi dual strand scoatedwith a rust-inhibiting rease,then s extruded n a seamlesslas-tic sheathing. ts applications n-clude slabs-on-grade, tructuralmats and raf t foot ings, c ircular

f luid tanks, and tilt-up walls. Be-cause he unbonded tendon is smallin di ameter as well as ight and flex-ible. it can make maximum use ofthe entire depth of a concrete slab,making it particularly well suited tothin slabs. and the time and cost as-sociated with grout ing is el imi-nated.With unbonded PT construction.the high-strength strand is placed ina draped configuration within theslab depth, usually with high pointsat supports and low points betweensupports (F ig. l). The concrete sthen placed, and after suff icientcuring [from 1500 o 2500 psi (10.317.2 MPa), depending on the an-chorage systeml, the steel strand isstressedagainst the concrete, thenlocked off under stressat end or in-termediate anchor points (Fig. 2) .In addition to inhibiting rust, thegrease around the strand reducesfriction to eliminate an y bond be-tween th e plastic an d the steel, al -lowing the strand to slide freely andmore uniformly distribute the ten-sile stressesn the strand.

Structural ehaviorPreviously, it was believed that ten-don placement into column andmiddle strips was required for a PTslab to behave as a flat plate. Dis-tribution of forces from 50 to 100percentto the column strips, and upto 50 percent in the middle strips,have been used successfully.Suchdistributions were normally adapta-tions of recognized values for regu-lar reinforced concrete design fac-tors. These arrangements produceda two-way weave that requi red

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carefu l sequencing procedures,greatly reducing productivity in ten-don placement an d increasing thesystem'soverall cost.More recent ly, tests and in-si tuapplications have shown that a PTslab behaves as a flat plate regard-lessof the tendon placement.' No tonly were the sequencedPT slabstime consuming to build, but theirdistributions failed to recognize hattendons exert downward forces atthe high points jo in ing parabol icprofi les.Tendon high points should be, asnearly as possible, resistedby col-umns, wal ls, or upward tendonforces to achieve minimum deflec-tions and maximum shear capaci-ties. This implies that all tendons inone direction should be placed overor immediately adjacent to columnlocat ions, and the tendons in theperpendicular direction should beuni formly spaced across the baywidth. This tendon placement is re-ferred to as a banded system. Sucha statically rational, yet simple andelegant, distribution of tendons isespecially advantageous n allowingthe easy tracing of load paths tosupports in slabs with irregular col-umn layouts (Fig. 3) .Apart from its obvious structuraladvantage, the banded tendon ar-rangement simplif ies constructionsequencing.First, all of the bandedtendons in one direction are placed,then all of the uniform tendons inthe perpendicular direct ion areplaced. No weaving of tendons isrequired, reducing abor costs. Sinceits introduction, the banded systemof tendon placement has gainedwide acceptance and preferenceover its sequencedcounterpart andis now used almost universally forPT concrete floor slabs.

Precut tendons usual ly areshipped to the site in coils of about5 ft (1.5 m) in diameter. Each ten-don is individually marked, clearlyidentif ing its location in the slab.Tendons longer than about 100 ft(30 m) usually require stressingatintermediate points along theirlength. Dead end anchorages arelocked into place at the shop andnailed to the slab edge orm on site.Intermediate and stressinsen d an -

choragesar e placed loosely on thecable (a t the approximate locationrequired) to allow exact placementon site. To minimize slab shorten-ing, and to limit friction losses, hemaximum distance between con-struction joints in the slab shouldnot exceedabout 150 ft (45 m).As mentioned previously, withthe banded system, all the tendonsin one direction are placed first over(or adjacent to) the columns. Thenthe tendons are placed uniformly inthe other direction, spacedat about6 to 8 times the slab thickness. Theconcrete is placed once all of theslab reinforcement is in place an dsecurely tied. Any admixtures non-injurious to steelmay be used n theconcrete.

The concrete forms are lef t inplace unt i l stressing s complete.Unlike RR s labs, in which deflec-tion must occur to activate the re-inforcement. the reinforcement isactive in PT slabs, usually provid-ing uplift forces between supports.Consequently, engineers and con-tractors have found reshoring of PTslabs o be lessonerous than for RRslabs.Approximate span,/depth ratiosfound to provide both economy andadequate performance for memberswith l ive load less than dead loadare: rone way slabtwo way slabtwo way slabwith drop panel(minimumdrop L/6 eachway)50two way slabwithtwo way beams 55waffleslab,5 ft (1.5m)square rid 35beamsb-h/3 20beamsb=3h 30ApplicationsFlat plates and s labs employing PTreinforcement have been used n of-fices, apartments, condominiums,and parking structures throughoutNorth America, where several mil-l ion square feet of post-tensionedflat plates are completed eachyear.2Surveys carr ied out by the Post -Tensioning Institute indicate thatover 250 million ft'z 23 million m'?)

of PT slabswerebuilt between 967and 1976. 'The longerspans ossiblewithPT reinforcement rovide he ad-vantage f additionalversatility ncolumn ayout. In addition, he useof PT reinforcement s not limitedto regularor orthogonalplacement,but is adaptable to ir regular lyshaped loor plans.Most modern parking structuresare built of cast-in-placeoncrete.When thesestructures re properlydetai ledand reinforcedwith PTtendons. cracks associatedwithbendingmomentscan be either re-duced or eliminated,helping o re-lievesomeof the problemshistori-cally associated ith deck deterio-ration.Post-tensioning as been oundto be economicalor slabs,beams,transfergirders,and foundations,as well as for a variety of other ap-plications. t hasbeensuccessfullyused n both low- and high-r isestructures, ncluding apartmentsand condominiums, ospitals, om-mercial buildings, ndustrial build-ings,and parking acilities.

DiscussionDuring the mid-1950s, when PT wasfirst being used n the United States,little was known of the behavior ofPT reinforced slabs. Consequently,the analysismethods were awkwardand time consuming, and the designwas conservat ive. The advent ofT.Y. Li n's load-bala ncing tech-nique, introduced n 1963, emovedthe mystery of post-tensioning, andthe analysis of PT structures wasgreatly simplified.3The load-balanc ing method is themost widely used procedure for an-alyzing and designing PT struc-tures. To simplify matters even fur-ther, as with its RR concrete coun-terpart , computer programs arenow available for analyzing and de-signing PT slabs and beams. Al-though they are no substitute fo rexperience and sound engineeringjudgment, such programs make therepetitive and calculation-intensiveaspectsof PT analysis and designless edious.Even in areaswhere only one PTsupplier is available, it still is possi-

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Fig.1 - Typical lab section howing rapeof post-tensioning ablebetween upports.

Fig.2 - Anchorage eta i ls.Fig.3 - F latplatewith rregular o lumn ayout.Notetendon orceseasi ly raced o column ocat ions ithbanded ystemof tendonplacement.

ble to includePT concreten com-petitivebidding.Typically,PT sup-pliershave n-houseengineeringa-pabi l i t ies. n areasof l imited PTcompetition, he structurecan bedesignedn regular reinforcedcon-crete,with the bid package etup toaccept nbondedpost-tensioning san alternate o the regular rein-forced concrete labs.The PT pro-posalcan be set up so that the PTsupplier ncludes he analysisanddesignof the slab for post-tension-ing, as well as the material and la-bor for its construction.n th is way,an owner can automaticallybe as-sured hat the systemused s theleastexpensive vailablewithout n-curring he front-end cost of multi-pledesigns.Still, the reluctance f some ouseunbondedpost- tensioningsrooted not in a lack of suppliers rinsufficient amiliarity with PT sys-temsor their designprinciples,butin a concern bout he tensile orceslocked n unbondedpost-tensionedtendons.This concern akes hreemain forms.The first is a belief that the lossof one span of this cont inuousmembersystemwill precipitate hecol lapseof the ent ire structure.

Model testscarriedout in Switzer-land and the U.S., as well as acci-dentson buildingswith construc-tion in progress, aveshown hatthe PT tendonsdo not respond olocalizedareasof overloading,andthat PT slabsexhibit good struc-tural redundancy nd toughness.aFurther,a surveyof PT tendonper-formance n a varietyof structuresconcludeshat properlydetailed ndconstructedPT structuresexhibitexcellentperformancewith respectto durabilityagainst orrosion.5The second ommonconcernhatsurfaces bout PT slabs s that ofthe new opening equiredafter theslab s bui l t . The bandedsystemused n PT construct ion rovideslargespaces etweenendonswithinwhich o locatesmall openings.Ca-bleJocatingequipmenthelps o en-sure that the opening s locatedaway from tendons.Until recently,large openingssuch as are some-times required for ne w stairs be -tween loors presentedmore of aproblem,since endoncutting couldnot be avoided.But a procedurenow hasbeendevelopedor cuttinglarge openingsn PT slabs.6 hemethod s fast, relativel y simple,an d - most importantly - con-

trolled. It is possible becauseof anewly developed detensioning jackthat makes a previously dangerousundertaking a safe procedure.The third common issue raisedabout unbonded post - tensionedslabs s the question of what hap-pens f and when the structure needsto be demolished. Many have vi -sions of unbonded tendons shoot-ing out from the slab edges. n a re-port on the demolit ion of 100,000ft '? 10,000m2) of PT slab, only asmall percentage exited from theslab edge, and in no event was theexit more than 18 in. (450 mm). 'The conclusion was that an un-bonded PT slab can be demolishedsafely and economically.Thus, the most common prob-lems and concerns associatedwithPT slabs have been either resolvedor shown to be unwarranted.2'6-e*AdvantagesOne of the main benef i ts of un-bonded post-tensioning s the abil-

*Aalami, Bijan O., and Barth, Florian G.,"Restraint Craks and their Mitigation inUnbonded Post Tensioned Building Structures,"ACI Convention presentation, Baltimore,Marvland. Nov. 1986.

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ity to achieveongerclear spans. fthe useor layout doesno t permitlongerspans, hen usingPT resultsin a reduct ion of both concretequant i ty and reinforcing steel.Thinner slabsproducea decreaseddead oad and a possible onse-quent eduction n foundationsize.Thinner slabsalsoproducea de-creasedloor- to- f loor height , re-sulting n an overallshorterbuild-ing; thus, exteriorskin,verticalpip-ing, ducting, and conduit costsarereduced. n addition, educed loor-to-floor heightsmay make t possi-ble to squeezen an extra loor.Forming costscan sometimes eloweredas a resultof the reduction

(o r removal)of drops and beams.Becauseendons tressinganoccurat relatively ow concrete trengths,earlier ormwork stripping s possi-ble, and with it comes asterplac-ing cycles,r0hichcan ranslatentodecreasedorrowingcostsdue oearlierproject completion.All oftheseactorspoint to a savingn theoverallcostof a building."Cost comparisonIn the final analysis, ard costsplaya significant ole in the decision ouse or not use a particular system.To carryout a relativecostcompar-ison betweenRR and PT concrete.computer uns were carriedout on

a numberof slabs,with spans ary-ing from 18 o 40 t (5.5 o 12.2m),fo r residential,office, an d retailloading. The cost of framed con-creteslabsconsisted f the con-crete. einforcement nd formworkcomponents. he programs sed orthe both the RR and PT concreteanalysis re basedon the equivalentframemethod.In the analysis,4000psi (27.6MPa) concrete was assumedthroughout.A reinforcement overof I in. (25 mm) was used or theRR slabs, and 3Ain . (19 mm) forthe PT sl abs. The comparisonsmadeare shown n Table .The material quantitiesobtained

(6) (8) (10)o RRStab o prsrab Span ft (m)

Fig.4 - Costversus pan or RRandPTslabs.Table1 - Slab ypesused o compare osts orRRversusPTconcrete

Table2 - Unit cost values or RR/PT labcomparison

ORRSIab EPTSIab Span ti (m)

Fig.5 - Costversus pan or RRandPTslabs.

E

6

TWO WAY SLAB

(6)o RRSlab E PTSlab

Fig.6 - Costversusspan or RRan d PTslabs

($75)

($50)

Occupancy Framingmethod

Spanvariation,ft(m)Liveload,ps f(kN,/m')

Superimposeddead oad,ps f(kN/m')Residential One-wayslab 18-40(5.5 12.2) 40( l .e) 25(1.2)Office Two-way slab l8-405.5 r2.2 50(2.4) 25(1.2)Retail Two-way slab 1 8- 40(5.5 12.2) 100(4.8) a


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were for a typical interior bay, sincethe exterior bay is normal ly af -fected by the particular buildingconfiguration and the exterior skinused. Also, square bays were as-sumed for the two-way slabs.The unit cost values used for thecompar ison are show in Table 2.The prices include material and la-bor for supplying an d placing th ecomponents listed. Thesevalues areconsideredvalid at the time of writ-ing and for the geographical areawith which the author is familiar.The resul ts of the compar isonsfor one- and two-way slabs weregraphed, with cost per square footversus span (Fig. 4 through 6), toprovide the information in an easilydigestible form.* The results consis-tentlv show that PT concrete slabs+Unit quantity tables that allow comparisonsbased on individual experience and knowledge areavailable from the author at Prosum Engineering,Ltd., 56 Goddard St., North York, Ontario M3 H5E2, Canada.

offer a sizeable ost saving elativeto their RR concrete ounterparts.References

1. Design of Post-Tensioned Slabs, Post-Tensioning Insti tute, Phoenix, Arizona,1977.2. Barth, F lor ian G., and Aalami, Bi janO., "Contro l led Demolit ion of an Un-bonded Post-Tensioned Concrete Slab,"Post-Tensioning Insti tute, Phoenix, Ari-zona, 1989.3. L in, T. Y., "Load Balancing Methodfor the Design and Analysis of PrestressedConcrete Structures," ACI JounN.cr, Pro-ceedings V. 60, No. 6, June 1963, pp . 719-742.4. Freyermuth, Cli f f , L., "Strucutra l In -tegrity of Buildings Constructed with Un-bonded Tendons," Concrete Internatio nal:Design & Construction, V. 11, No. 3, Mar.1989,pp . 56-63.5. Schupack, Morr is, "A Survey of th eDurability Performance of Post-TensioningTendons," Post-Tensioning Insti tute, Jan.I 978.6. F ischl i , Franz; Grimm, Marcel; andMart i , Peter, "Cutt ing Openings in Post-Tensioned Floor Slabs." Concrete Internu-tional: Design & Construction, V. 10, No. 2,Feb. 1988,pp.45-47.

7. Chacos, Gregory P. , "Resolut ion ofField Problems with Unbonded Single StrandTendons," Concrete Internotional: Design &Construction, V. 10, No. 2, Feb. 1988, pp .40-44.8. Falconer, Danie l W. , "Tips for Post-Tensioning," Concrete Internotional: Design& Construction, V. 10, No. 2, Feb. 1988,pp .36-39.9. Field Procedures Manual for UnbondedSingle Strand Tendons, Post-Tensioning In-stitute, Phoenix, Arizona, 1989.10 . Kenny, Al lan R., and Song, JaeWoong, "Post-Tensioning Alternative Re-duces Construction Time," Concrete Inter-national: Design & Construction, V. 10, No.2, Feb. 1988,pp.48-52.11 . "Multistorey Office Building - Post-Tensioning Saves $% Mil l ion," Port landCement Association, Skokie, Illinois, 1975.Selected or reader interest bv the editors.

ACI memberAngelo Mattacchioneis president f ProsumEngineer ing,Ltd.,NorthYork,Ontario, anada. eis a member f ACICommittees 18,Useof Computer s, nd 340,DesignAids or ACI Bui ld ing odes.

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unbond pt slab economic alt

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