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How does the pre-stressing force transmit in post-tensioning beams/slabs?

  • 1.  How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-17-2018 02:26 PM
    Edited by Tirza Austin 08-17-2018 04:34 PM
    It's known that for the pre-tensioned members, the tendons are stressed first and soon the concrete is placed which leads to a (what I call) perfect bonding between the tendon and the surrounding concrete. This achieved bonding is the main key to transfer the stresses. In case of post-tensioning, the tendon is placed through ducts and stressed later after the concrete is poured, which means there's no bond (at least not a perfect one) between the tendon and concrete. So how would the pre-stress transmission work here? I understand that for bonded tendons the grout will help in building that friction between the two surfaces and hence transferring the stresses, but what about the un-bonded ones? Why would we use the un-bonded system?

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    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    +86 183 5193 3411
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  • 2.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-19-2018 12:50 AM
    Edited by Tirza Austin 08-19-2018 12:50 AM
    Think of the post-tensioning tendon and its end anchorages as a large "clamp" that compresses the concrete element.  Remember that the reason for pre-stressing concrete is to impart compression to offset tension forces (and therefore stresses) imparted by service loads on the concrete member.  Pre-tensioning and post-tensioning both are pre-stressing.  Pre-tensioning is impractical for very large cast-in-place elements.

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    Richard Lenz, Life M. ASCE
    Retired
    West Chester OH

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  • 3.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?
    Best Answer

    Posted 08-19-2018 02:45 PM
    Edited by Muhannad Husain 09-05-2018 11:03 AM
    I feel that some clarification is in order:


    Pre-tensioning (commonly referred to as "Prestressing") needs "bulkheads" to support the prestressing jacks, since the concrete is not there yet. Since it's unlikely that such bulkheads can be installed on the jobsite, they are restricted to fixed locations at precast concrete plants. Strands are usually 1/2" or 5/8" diameter, of 7-twisted wires, 270 ksi ultimate strength. "Low Relaxation" strands are most common for minimizing priestess losses. Prestress is usually released when concrete strength reaches 70% of its 28-day strength. To accelerate that in a plant, steam-curing is used. The prestressing force in a strand is developed by friction. This together with the "bursting effect" at the strand end, are usually quite modest given the small diameter of strands. I can't give the Precast/ Prestressed Concrete industry justice in such few lines but I can refer you to the website of PCI, the "Precast/ Prestressed Concrete Institute" of USA (www.pci.org). If interested in metric version, you can refer to its Canadian sister CPCI (www.cpci.ca). They both have great literature.

    Post-Tensioning is mostly done on job sites, even though it is used occasionally in precast concrete plants. Its scale varies widely from a 1/2" dia. Monostrand with, to a tendon of dozens of strands. PT relies only on the end jack being kept in place after being "seeded" (its wedges being pushed in the end cone after jacking is completed). PT strands as well as PT tendons come in lubricated sheaths to minimize losses of prestress due to friction. Sheaths allow for flexible profiles that fit moment diagrams best. Monostrands are very common in light PT of cast-in-place concrete slabs. (Also, I used it in Architectural precast concrete). No grouting can be injected in such tight-fitting sheaths. Large-scale heavy PT, using tendons can accommodate sheath grouting after PT completion, but the PT force is still maintained by the end jack; we can't rely on grouting for development of tendons. Due to the total reliance on jacks at the ends, these jacks should be protected in patched recesses. Bursting end zones are particularly critical in PT. While the PT hardware, "Trumpets" and their surrounding reinforcement are designed to mitigate this issue, heavy PT needs special attention to the design of those bursting zones where every possible mode of failure needs to be identified and accounted for. Applications of PT include, in addition to CIP structures, segmental counter-cast precast concrete held together by heavy PT, usually in bridges. Also used at a stadium roof: Zayed Sports City in UAE. More info on PTI "Post-Tensioning Institute" website: Post-Tensioning Institute - post-tensioning research, code, development and marketing

    Post-tensioning remove preview
    Post-Tensioning Institute - post-tensioning research, code, development and marketing
    PTI is a professional association dedicated to research, specifications, design recommendations and providing information on post-tensioning and construction technology through publications and education.
    View this on Post-tensioning >





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    Neil Kazen,
    M.Eng., M.Sc., P.Eng..
    F.ASCE, F.CPCI, Award of Merit- PCI
    Retired Structural Engineering Manager
    Toronto ON
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  • 4.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-19-2018 12:50 AM
    Edited by Tirza Austin 08-19-2018 12:49 AM
    Post-tensioning forces are transferred to the concrete by anchorages at the ends of the tendons.  The end that the tendons are stressed from typically uses a steel chuck bearing on a cast-in steel anchorage.  The opposite end may be anchored in by a similar setup or may have a flared tendon end cast into the concrete.

    Here are a couple of material suppliers that have some good info on the systems.

    VSL | Post-tensioning strand systems | Post tension
    Vsl remove preview
    VSL | Post-tensioning strand systems | Post tension
    VSL designs, manufactures and installs durable, state-of-the-art post-tensioning systems that comply with international standards and approval guidelines for both new and existing structures.
    View this on Vsl >



    Post-Tensioning
    Dywidag-systems remove preview
    Post-Tensioning
    DYWIDAG Post-Tensioning Systems - Reliability and a long Service Life for your Project. For decades, we have been your competent contact for bridge, industrial and tank structures in Construction Engineering.
    View this on Dywidag-systems >



    Another good resource is the Post Tensioning Institute.  Post-Tensioning Institute - post-tensioning research, code, development and marketing
    Post-tensioning remove preview
    Post-Tensioning Institute - post-tensioning research, code, development and marketing
    PTI is a professional association dedicated to research, specifications, design recommendations and providing information on post-tensioning and construction technology through publications and education.
    View this on Post-tensioning >



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    Robert Holland P.E., M.ASCE
    Engineer
    Oldcastle Precast
    Spokane Valley WA
    (509) 536-3349
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  • 5.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-25-2018 01:34 PM

    Thank you so much for the valuable comments. I may conclude that:

     

    -post-tensioned members basically do transfer stresses by means of wedge actions in end zones, regardless the adoption of bonded or unbonded tendons.

    -grouting is introduced in bonded tendons as a way of protection only.

     

    Am I right?

     

    Since the difference between bonded and unbonded is the grout injection, does that mean for two identical beams (same material properties, same prestress applied, etc, but one with bonded and the other with unbonded tendons) the stress distribution and prestress transfer would be nearly the same and follow the same trend?



    ------------------------------
    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    86183 51933411
    ------------------------------



  • 6.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-26-2018 10:35 PM
    The "bonded" is used only in prestressing strands.

    Grouting of PT tendon sheaths does not constitute "bonding" for design purposes, even though it has some helpful bonding effect.

    A similar phenomena, where a helpful material is not counted in design, is caulking in the joints of architectural precast concrete panels: when it hardens it can acquire great strength, yet we don't count it in design; simply because we can't quantize its effect. I remember a situation where we were removing such a panel: after hooking it to crane, it's hardware connections were cut, the crane operator tried lifting it assuming that caulking was nothing to worry about. The result was that the panel didn't move but the crane fell. A surprising demonstration of the strength of aged caulking that we couldn't quantize. Think of sheath grouting as similar.

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    Neil Kazen F.ASCE
    FCPCI, Award of Merit- PCI
    Retired tructural Design Manager
    Toronto ON
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  • 7.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-27-2018 02:35 PM
    Edited by Tirza Austin 08-27-2018 02:35 PM
    Post Tension Institute (PTI) specifies both bonded and unbonded as does ACI 423 in their current guides. A bonded tendon still transfers force from the grout to the duct to the concrete, whereas an unbonded tendon that is damaged is completely lost.

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    Martin Fradua P.E., F.ASCE
    Vice President
    Huntington NY
    (631)470-2019
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  • 8.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-27-2018 11:28 AM
    Edited by Tirza Austin 08-27-2018 11:30 AM
    You are correct that unbonded post-tensioning has forces transferred through wedge action.  This is also true for bonded post tensioning.  The bonding does several things, provides protection to the tendons and if the tendons break the effect of the breakage is localized to the area of the break.  The grouting in the tubes allow the force from the tendons to transfer through the grout and then the duct into the concrete.  The transfer of stressing forces from a broken tendon through the bonded duct is similar to the a pre-stress strand into concrete.

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    David Thompson P.E., M.ASCE
    Principal
    KTA Structural Engineers Ltd.
    Calgary AB
    (403) 246-8827
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  • 9.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-28-2018 10:26 PM
    Edited by Tirza Austin 08-28-2018 10:25 PM

    I’m concerned with the statement, "and if the tendons break the effect of the breakage is localized to the area of the break.  The grouting in the tubes allow the force from the tendons to transfer through the grout and then the duct into the concrete. "

    What's the development length for a tendon with 200 tonnes (440 kips) PT? How about 900 tonnes (2000 kips), my heaviest PT to date?


    Further, in a tendon of several strands, not all strands are in full touch with the grout.


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    Neil Kazen F.ASCE
    Structural Design Manager Transportation Division
    Toronto ON
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  • 10.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-29-2018 11:44 AM
    Edited by Tirza Austin 08-29-2018 11:43 AM