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STRUCTURE magazine April 20108

Post -Tensioned Slabs on GroundPart 2: Specific Design ConsiderationsBy Bryan Allred, S.E.

The January 2010 issue of StructureMagazine contained a general overviewof the design and construction of post-tensioned slab on ground foundations.This article will focuses on specific engi-neering items that occur when designing

these types of foundations. As describedpreviously, post-tensioned slabs on gradeare primarily designed to support residen-tial and light industrial construction thatis on expansive or compressible soil. Thefoundations can be designed per the Post-Tensioning Institute (PTI) method to be aribbed or uniform thickness foundation.

Ribbed FoundationsA typical ribbed foundation will havea 5-inch thick slab, with interior and

exterior footings that extend from one endof the foundation to the other (Figure 1).The layout of the footings (ribs) will giveit an exaggerated waffle slab appearanceif it could be viewed from the soilsperspective. Due to their depth, thefootings will provide the vast majorityof the foundations section modulusand moment of inertia, which are thekey parameters in limiting the flexuralstresses and deflections. Although it maybe economically advantageous to havea few very deep footings to generatethe same elastic section properties, theperformance of the foundation maysuffer with large gaps between ribs. ThePTI method limits the maximum spacingof footings to be 15 feet, and requires thespacing of adjacent footings to be 20%of each other. The spacing limitationsare intended to have the footings closeenough such that the foundation canrespond as having a consistent stiffnessacross its cross section, rather than havinglocalized areas of large stiffness that areconnected by a relatively thin slab. Intypical structures, the footings are around10 to 12 feet on center in each direction.

A tighter rib spacing will also minimizefooting depth and width. In the authorsopinion, the footings should be locatedunder the lateral system and the loadbearing elements (walls, post or columns).In addition to providing vertical andlateral support for the structure, thefootings location will be linked to thearchitectural plans which will minimizedimensional discrepancies in the betweenthe structural and architectural drawings.

The footing width and depth will dependon the specific site conditions and the loadof the structure, but they are typicallyaround 18 to 24 inches deep (includingthe slab thickness) and 12 inches wide.Footings that are wider than 14 inches canbe constructed, but the width is limited to14 inches for the numerical design.Footings of different depths can be

used, but the ratio of largest to smallestmust be kept within 1.2. This is mosttypically seen where deeper footings areused on the perimeter due to larger postloads/hold downs forces, or specific embed-ment requirements of the soils report.

Although the foundation wil l benefitfrom deeper footings, the code limits thenumerical design to the 1.2 ratio. Usingvery deep exterior footings to generatesection proprieties that satisfy stress lim-its, such that interior footings are notrequired or are very shallow, is not theintent of the methodology.Instead of adding ribs for bearing wall

loads, the PTI method contains a pro-cedure for the slab to act as a footing. Atypical post-tensioned 5-inch slab with acompressive strength of 3000 psi and aprecompression force of 50 pounds persquare inch (code minimum) can supporta 1,900 pound per linear foot bearing wall.This capacity can be increased with thickerslabs, higher strength concrete or a largerprecompression force from the strands. Inmost residential construction, footings are

not required to support the bearing wloads. The potential to have the slab aca footing is also useful in home remoor tenant improvements, since the requment for new footings can be minimizThe same philosophy can be used for slab to resist post loads. For most ccompliant designs, a post-tensioned scan comfortably resist 1,000 poundsload for each inch of thickness.For typical single family home c

struction, the tendons in a ribbed systare approximately 3 to 4 feet on cenin each direction. This spacing alloeasier installation and inspection whminimizing the potential of field persnel damaging the strands or pushthem off their supports. In addition, relatively large spacing typically provisufficient room for plumbing and otpenetrations without modification to tendons. The spacing of the tendon is required to be placed at a specific spaciVariations in the tendon locations, to avpenetrations, re-entrant corners or hdowns, are permitted provided the spacbetween adjacent strands is less than 6 fIf a spacing of 6 feet is required, additiorebar may be required for crack conand continuity of the foundation.The number of strands is primarily

fected by the desired precompressforce and the sub grade friction resistanFor the same site conditions and cstruction, a larger footprint foundat

Figure 1: Ribbed Post-Tensioned Foundation.

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Figure 2: Uniform Thickness Post-Tensioned Foundation.

will have more tendons since the sub gradefriction force increases with size. The codeminimum precompression of 50 psi is calcu-lated at the middle of the foundation, wherethe affect of sub grade friction is the largest.For larger foundations, there will most likelybe a substantial difference between the pre-compression force at the edge of the foundationcompared to the middle.The tendons in the majority of post-

tensioned foundations are located in the center

of the slab and run flat from anchor to anchor.Care should be taken by the contractor anddeputy inspector to eliminate localized kinks(vertical and horizontal) in the strands whenthey extend over footings or where they arecurved to avoid penetrations. The strandsare intended primarily to provide a precom-pression force throughout the foundation toreduce flexural tension stresses. They are notdesigned as tension reinforcement that con-forms to the moment diagram as they wouldin elevated slab and beam design. The designof these foundations is based solely on allowable

stresses, so placing the tendons at differentlocations of the slab will not provide in anybenefit in the design. Balance loads createdby vertically draping the tendons arent re-quired since the structure is supported bythe soil. In addition, since the expansive soilmovement can occur in both vertical directions,

load balancing may help in one conditionwhile hurt the system in the other. Even

though load balancing is the primary benefitof post-tensioning in elevated slab and beamdesign, it should play no part in the designof these ground supported foundations. Insome extreme edge lift cases, specific tendonsare anchored at the mid-depth of the slaband immediately draped to the bottom of the

footing, where they extend across the foundauntil they are draped back up at the ot

end of the foundation. This profile will creadownturned force that is intended to coteract the force of the soil moving upwarThis type of design is primarily done in Te

where they have very large edge lift contions. Even with expansive soils, everythis larger in Texas.

continued on next p

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Uniform ThicknessThe uniform thickness option is designed

by converting the section properties of thecode compliant design ribbed foundationto a single thickness slab. The conversion isintended for the slab only, and does accountfor the presence of exterior footings. Usingthe section properties of the exterior footingto minimize the uniform slab thickness is notthe intent of the PTI method or the building

code. Without interior footings, the slab alonewill be required to provide the stiffness andstrength to resist expansive and compressiblesoil movement. Perimeter footings are oftenrequirements of the soils report or to resistlarge post or hold down loads, but since theirstiffness cannot effectively be distributed overthe entire foundation, their influence is to beignored in the conversion.Typical uniform thickness slabs are in the 8

to 12 inch range (Figure 2, page 9) and havebeen used to support up to 5 stories of woodframe construction. They typically have more

concrete when compared to a ribbed foundation,so more tendons are required to provide therequired precompression. Without interiorfootings, deepened sections of concrete maybe required to resist large post loads or shearwall hold downs. Depending on slab thick-ness, localized footings are often used underthe lateral system to satisfy the allowable soilpressure due to overturning.Regardless of the slab thickness, the tendons

are still located and anchored in the middleof slab. This layout can create slabs with 5to 6 inches of cover from the strands to the

top of the concrete. Although this would notbe permitted in elevated slabs, there is noadditional top rebar required in these types offoundations. Some engineers will place a gridof rebar or mesh in the top of the slab, but thisis not a requirement of the PTI method andmay be done simply for crack control. In thelarger foundation plates, tendons can have arequired spacing of 12 to 24 inches on center.To maximize the distance between strands, thetendons can be grouped into bundles of twosor threes. The bundled strands are placed sideby side, are typically tied together, and use

the same chairs or dobies for support. Theyare separated near the slab edge or stressinglocation to allow for the installation of theindividual anchors. In addition, this practice

will minimize having a series of tendons thatare varying lengths if the slab edge has anangled or saw tooth configuration.

Cost a Factor

For engineers and contractors that are new topost-tensioned foundations, a common questionis what type of foundation is best suited formy project? As in most situations in construc-tion, the contractors price is typically thedeciding factor. With the ribbed system, moretrenching is required, and this has been a costand time issue for some contractors. In addi-tion, the ribs require maintenance during theplacement of the reinforcing and the vaporretarded. Portions of the soil may drop into thetrench during construction, affecting the foot-ing depth and covering the rebar with dirt. The

uniform thickness foundations will have lesstrenching but more material costs in concreteand tendons. If you are dealing with methaneissues, the uniform thickness slab option istypically preferred since its more economicalto install the barrier relatively flat rather thanextending it through a series of overlappingfootings. In the authors experience, most singlefamily homes are constructed with a ribbedfoundation, while multi-level apartment/condominium projects are designed with auniform thickness system.

CorrosionFor post-tensioned slabs on grade, the soils

report should include information regard-ing the corrosiveness or the chloride contentof the site. For corrosive sites, encapsulatedtendons (Figure 3) are used to further protectthe strand, anchor and wedges from deteriora-tion. The strand, anchor and wedges are thesame between the standard and encapsulatedsystem; only the sheathing and the anchorcovering is changed. The anchor assembly isencased in a watertight connection betweenthe strand, sheathing, anchor and wedges.

Standard encapsulated systems are hydrostcally tested to verify water tightness. For reason, tears in the sheathing, regardlesslength need, to be repaired and anchors tarrive on site in a damaged condition nimmediate attention. Sealing the encapsulasystem after the tendons have been exposedan aggressive environment will only lock corrosive elements into the system, increasthe likelihood of damage. Unlike the sultable in ACI 318, there is no correspondcorrosive chart that identifies when encapsulatendons should be used. Per the PTI recomendations, if the soils report lists modeor above chloride content or if the replists the site as corrosive to ferrous metals other similar language), encapsulated tendshould be specified on the structural drawing

ShrinkageTo minimize shrinkage cracks, it is crit

that the tendons are stressed as soon as p

sible. The PTI design manual recommethe tendons be stressed within 10 daysplacing the concrete. The precompressfrom the strands is the primary crack conreinforcement in the foundation. The soothe strands are stressed, the sooner the slab resist the tensile stresses generated from shrinkage of the concrete. These foundati

will typically have very little rebar, so untiltendons are stressed, the concrete is essentiun-reinforced and prone to cracking. To furresist shrinkage cracking, some engineers wsaw cut the slab within the first 24 hours a

placing the concrete (Figure 4). The depth spacing of the saw cuts will vary dependon the slab thickness so the tendons will be damaged during construction. In adtion, the saw cuts need to be specified suthat the continuity of the slab is not signcantly impacted.

Figure 4: Saw Cuts on a Uniform Thickness Foundation.

Figure 3: Encapsulated Tendon with a Pocket Former.

Bryan Allred is a license structuralengineer and Vice President of SenecaStructural Engineering Inc. in LagunaHills CA. He can be reached via email aBryan@SenecaStructural.com.