Common PT Design and Construction Issues by Allred

8/12/2019 Common PT Design and Construction Issues by Allred

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Over the last two years, I have participated ia post-tensioned concrete design seminaseries for the Post-Tensioning Institute (PTI) thahas covered the majority of the U.S. From thesseminars, my own experience in engineering anobserving post-tensioned concrete, several desig

and construction issues appear to be consistenamong most engineers and contractors.

Balance LoadsWhen designing a post-tensioned slab o

beam, the first thing experienced post-tensioninengineers check is what balance load is beinproduced by the tendons. The balance loais the upward force created by the load in thtendons trying to straighten out from theconcave drape in the concrete. The balancload will minimize and often effectively remov

the dead weight of the concrete from the streand deflection calculations. With the concretdead weight removed, the long term deflectiomultipliers of 3 to 5 are irrelevant, since thinitial dead load deflection is basically zero.

A good balance load is typically between 7and 100 percent of the weight of the tributarstructural floor system. Superimposed dea(partitions, flooring, ceiling, mechanical, etcand live loads are not included in this percentagsince they will not be present at the time ostressing. Many new post-tensioning engineeover-look balance loads since they are not a codrequirement and low/high values will not generate red flags by most commercial software.

Applying more balance load than the systemweighs is called over balancing. This can occuon every post-tensioned project, but is typicallprevalent on slabs that will support largsuperimposed loads or slabs that are too thifor their respective spans. Over balancing caprovide a numerical solution to satisfy the allowable stress requirements, but may create otheissues. The problem occurs during stressin

when the tendons begin pushing up with morforce than the weight of tributary system. Thnet upward force can result in large tensiostresses at the bottom of the slab/column join

where there is typically little or no rebar, or it caactually lift up the slab. Unlike rebar, which onlactivates when loaded, too much balance loa(number of strands and/or drape) can significantly impact the slab.

Common Post-TensioningConstruction Issues& By Bryan Allred, S.E.

Figure 2:Slip providedby plastic

placed overthe masonrywall below to

prevent bond(See Figure

3 for slip ofwall dowel inthe masonry)

Figure 3: Slip detail using foam rubberaround masonry wall dowels

Figure 1: Slab crack due to the restraint

of the perimeter masonry walls


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STRUCTURE magazine July 2005

Slip ConnectionsA post-tensioned system will move ap-

proximately 20 to 30 percent more than con-ventionally reinforced concrete. A good ruleof thumb is that a post-tensioned system willroughly move about 1 inch for every 100 feetof slab that is not restrained by a lateral system.If the edge of the slab is 50 feet away fromthe nearest shear wall, this edge would move

about -inch. If this edge movement isprevented, the slab or restraining element willmost likely crack. Slab restraint is typicallycaused by concrete or masonry walls thatare hard connected at the perimeter of thestructure (Figure 1). In addition to havingmore movement, a post-tensioned slab willhave substantially less rebar than a conven-tional system, one of the main economicbenefits. Restraint cracks are often large andnoticeable since a post-tensioned slab doesnot have the excess rebar to minimize anddistribute cracking. Permanent slip details

are critical for the performance and aestheticsof post-tensioned concrete.Typical slip details utilize felt, building

paper or plastic to eliminate the bond of theslab to the walls (Figure 2). When rebar isrequired between the slab and the walls,pipe insulation or foam rubber surroundinga portion of the dowel can be used to allowrelative movement without activating thedowel in shear friction (Figure 3). Pour (alsocalled delay) strips are often used in post-tensioned buildings to minimize restraintissues and to create stressing locations. When

done properly, pour strips work very well,but they are only a temporary solution.Pour strips typically remain open forapproximately 45 days, however, the slabmay move for another year if not more. Ifpermanent slip details are not used, a pourstrip by itself will provide only a minoramount of relief.

Pour StripsPour strips are typically located at the

midspan or quarter point of the bay. Toprovide any crack control benefit, the slabs

on each side of the pour strip should becompletely separate. Any reinforcementcrossing the slab edges will act as a tensiontie, restrain the relative movement of thetwo slabs and will most likely cause cracking.All rebar and post-tensioning, includingdrag and chord reinforcement, are to belapped inside the pour strips (Figure 4).

Contractors should pay special attention toverify if the engineer requires the edges of thepour strip to remain fully shored after thetendons have been stressed. The confusion

Figure 4: All reinforcing islapped inside the pour strip

occurs because, after a successful stressing,the majority of the slab (except the pourstrip) is structurally stable and the forms andshores can be removed. Without re-shores,the midspan pour strips can create largecantilevers resulting in significant deflectionsand moments that were never intended orreinforced by the engineer.

Tendons in Two Way SlabsTwo way post-tensioned slabs are con-

structed with tendons grouped together in a4- to 5-foot wide band in one direction, anduniformly spaced tendons in the other. Atthe job site, the system resembles a one wayslab (uniform tendons) that is supported byan embedded slab beam (banded tendons)that runs along the column line (Figure 5).The most effective and efficient layout forthe two way system is to locate all of the

banded tendons in the upper most layer(least amount of top cover) at the columns.With this layout, all of the banded tendonswill have their maximum effective depth,and only two of the uniform strands across

the entire tributary width will have a duction. This is the closest way to achiequal strength in both directions withadditional strands or rebar.

A common placing error occurs when all of the banded tendons are placed belthe two column uniform strands. If occurs, the entire banded direction will lbetween - to 1-inch of its effective dept(Figure 6). For an 8-inch slab, a loss of 1-in

of effective depth at each column will rein approximately a 17 percent loss in balaload and moment capacity.

Increased Cover for Fire ProtectioPost-tensioning strands have differ

cover requirements, depending on if they located in restrained or unrestrained spaThe reference to restrained or unrestrained nothing to do with the type or location of lateral system. It is a function of the expanspotential of the slab due to fire. An un

strained span is the first or last span in direction of the tendon.

For a 3 hour fire rating, per table 72of the IBC, the minimum bottom cofor the tendons is 2 inches, while it is o1 inch for rebar. This significant differein cover is frequently missed and is a dircode violation. One retrofit option is to hplaster or other fire rated material applto the slab/beam around the low pointthe tendon to increase the fire rating. In potensioned slabs or beams, the cover to strands should typically be greater than rebar in the end (unrestrained) bays.

Water Proofi ngCompressing concrete by using post-t

sioning does not make the concrete waproof. Concrete by nature is a porous mateand adding some amount of compressdoes not suddenly make it non-porous. Ptensioning will probably enhance the natu

water tightness that concrete possess, buwill never be water proof.

Figure 5: Two way slab reinforcing

Figure 6: Banded tendonsplaced too low at the column


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I have heard several engineers and con-tractors make the argument that water proofingat roof slabs is not required with a post-tensioned system. This argument is wrong andcan open the engineer to increased liabilityfor water intrusion issues. An argument caneasily be made that more post-tensioningactually makes the system less water proof.For higher values of pre-compression, theslab will experience larger movementswhich may induce restraint cracks. Nomatter how much reinforcement is put intothe slab, a crack is definitely not water proof.

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Water intrusion can beseen though the crack inthe post-tensioned roofslab in Figure 1.

Concrete Blow OutsEach tendon, whether

there in a small slab ongrade or a 30-story hotel,

will be loaded to approx-

imately 33,000 poundsduring stressing. This forceis then transferred to theconcrete through the bearingof the ductile iron castinganchor. If the concrete is not

well vibrated, penetrationsare located in front of the anchors (Figure 7),or congested rebar prevents a uniform bearingsurface, a concrete blow out is likely (Figure8). The concrete will literally explode asthe anchors crush into the slab or beam,

which then can cause the hydraulic jack to

move suddenly and violently. For this reason,only trained professionals should operate orbe anywhere near the jack during stressing.If the penetrations can not be relocatedaway from the anchors, steel pipes shouldbe added around the openings to resist theanchorage force.

Drilling into a Post-Tensioned Sla

There are several myths about potensioned concrete, but the most prevalone I hear is that it is practically impossito drill into a post-tensioned slab. Timpossibility comes from the unknolocation of the tendons and anchors. general, as long the tendons and the concrin front of the anchors are not damagdrilling into a post-tensioned slab is a faroutine issue.

Existing tendons can be located simby the use of a pacometer (hand held mdetector) or an x-ray. With the x-ray in hathe technician should be able to draw tendon locations directly on the concr


Figure 8: Concrete blow out at a congestedanchorage zone

Figure 7: A penetration in front of the anchorswill most likely cause a concrete blow out

BRIDGES

MINING SYSTEMS

TANKS

DOMESTIC BUILDINGS

COMMERCIALBUILDINGS

EXCAVATIONS

SLOPE STABILIZATION

TUNNELING

HYDRO AND

MARINE STRUCTURES

DYWIDAG Post-Tensioning Systems

Multistrand Post-TensioningSystems

Bar Post-Tensioning Systems

DYNA Bond Stay CableSystems

DYNA Grip Stay CableSystems

Engineering and Design

Construction Methods

Form Travellers

Stay Cable Testing

Supply and Installation

System 100 - FDOTcompliance pending

www.dsiamerica.com

525 Wanaque AvenuePompton Lakes, NJ 07442Phone: (973) 831-6560Fax: (973) 831-6503E-mail: [email protected]

320 Marmon Dr.Bolingbrook, IL 60440Phone: (630) 972-1100Fax: (630) 972-9604E-mail: [email protected]

4732 Stone DriveTucker, GA 30084Phone: (770) 491-3790Fax: (770) 938-1219E-mail: [email protected]

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DYWIDAG-SYSTEMS INTERNATIONAL USA INC.

Post-Tensioning & Reinforcement

System 100

System 100System 100


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surface. I have personally used a $40 metal

detector and located tendons inside a 12-inchwide beam. For new office buildings wheretenants frequently change, we recommendmarking one side of the slab so that futuretenants will know exactly where the strands andanchors were placed (Figure 9).

Smooth Tendon ProfilesThe tendons should be draped vertically and

horizontally in a smooth gradual profile.Tendon discontinuities will attempt tostraighten out during stressing and produce

localized point loads on the slab. The tendonsin Figure 10 will create a downward forcethat is no where near a band or a verticalsupport. Instead of picking up the concrete,the reverse curvature will push down intothe lightly reinforced slab below, most likelycausing cracks or a small blow out. Thesetendon kinks are typically caused by the tendonsbeing kicked off their chairs, wrong chairsbeing used, or the strand being tied off to a pieceof rebar that has been moved after the tendonwas secured.

Figure 9: Tendons are marked to facilitate future tenant improvements

The above list of issues is certainly far fromcomplete, but represents common items that Ifrequently encounter. With the advancement

of computer software, the calculation part ofpost-tensioned concrete has becoming moreaccurate, efficient and visually stunning.Even with these improvements, the softwarestill does indicate where the system will lockup and where cracks are likely to occur. Gooddetailing and layout are critical for successfulpost-tensioned concrete. If you have questionson any aspect of post-tensioning, I recommendusing the discussion forum on the PTI web page(www.post-tensioning.org). Most questionswill receive several responses and hopefully willanswer your question.

Bryan Allred, S.E. is Vice President of Seneca Structural Engineering, Inc. in Laguna HilCalifornia. Bryan specializes in reinforced concrete buildings utilizing post-tensioned floosystems, the retrofit of existing buildings using external post-tensioning and post-tensioned slabon grade. He can be reached at [email protected]

Figure 10: Reverse curvature of the tendons willpush down on the slab

Common PT Design and Construction Issues by Allred

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