Original Message:Sent: 09-05- 11:56From: Muhannad HusainSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?Thanks a lot. Quite comprehensive answers.About the post-tensioning from both sides of a slab, I remember a PT practitioner told me generally it's preferable to stress from both ends (two live ends) had the tendon length exceeded 35 meters long. The longer the tendon, the more friction losses take place. One other reason to stress from both ends is the wobbling effect (as illustrated in the attached figure).------------------------------Muhannad Husain S.M.ASCEResearcherHohai UniversityNanjing Original Message:Sent: 08-31- 05:45From: Nitin BaraskarSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?The pre-stressing forces in post-tensioning system is transferred at cable ends locations only where they are locked in beams/slabs for resisting tension in the section at given location along the member span. Examples:a) Bonded cables system (cable ducts are grouted with non-shrinkable cement grout). In bonded system cables are locked at one end termed as dead end if embedded in concrete or fixed end if cables are held in steel plate using steel conical wedges at fixing point. The cables profile are arranged within the section depth to resist the maximum design flexure and tension at given section. Then, cables are pulled from free end would be either one end or both ends to generate the required resisting tension. After attaining the desired forces cables are locked at ends called as anchorage end. The anchorage end/zone are designed for extremely high compression forces generated due to cables pull and locked at ends. The cables are allowed to locked during this stage, meanwhile different losses occurred like slip at wedge, friction offered to cables from ducts and cable profile, etc. Once, cables are locked ducts are grouted to prevent further losses in forces and to protect cables from corrosion.b) Non-bonded system: cable ducts are not grouted, the forces get transferred to fixed ends where cables are locked.In case, cables breakage, the provision of dummy cables (4% of total number of cables) is provided to compensate unexpected loss of forces in both bonded and non-bonded pre-stressing post-tension systems.------------------------------Nitin Baraskar A.M.ASCESenior EngineerBechtel India Private LimitedNew Delhi ND+Original Message:Sent: 08-30- 07:54From: Allen HulshizerSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?Actually this is a general comment regarding Post-Tensioning [PT].In Nuclear Containment PT construction there is a requirement to check the existing tension load at given periods.Also, from time to time the PT is released and a section of the wall removed for Steam-Generator replacement and then Re-tensioned.In a number of cases, when the large wall section has been removed, significant delamination has been discovered. Crystal River was shut down as a result of not being able to resolve the further delamination when re-tensioned.Any comments on the delamination occurrence and a solution?------------------------------Allen Hulshizer P.E., F.ASCE, FACIConsulting Structural EngineerChalfont PA(215)Original Message:Sent: 08-29- 11:32From: David ThompsonSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?I would agree with your concern about transferring tensioning force through friction with large tendons. You just have to look at slippage of strands in pre-stressing after the strands are cut.There are grouted ducts used in slabs where strands or small tendons are used that my statement would apply.Our firm has not used unbonded post-tensioning since the 's. Our concern was the sensitivity of the tensioning system to construction methods and quality leading to corrosion, but that is a different topic------------------------------David Thompson P.E., M.ASCEPrincipalKTA Structural Engineers Ltd.Calgary AB(403) 246-Original Message:Sent: 08-28- 12:27From: Neil KazenSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?
Link to Ruiyi
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 ( kips), my heaviest PT to date?
Further, in a tendon of several strands, not all strands are in full touch with the grout.
------------------------------Neil Kazen F.ASCEStructural Design Manager Transportation DivisionToronto ONOriginal Message:Sent: 08-27- 11:09From: David ThompsonSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?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.------------------------------David Thompson P.E., M.ASCEPrincipalKTA Structural Engineers Ltd.Calgary AB(403) 246-Original Message:Sent: 08-24- 06:56From: Muhannad HusainSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?
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.ASCEResearcherHohai UniversityNanjing Original Message:Sent: 08-17- 14:14From: Muhannad HusainSubject: How does the pre-stressing force transmit in post-tensioning beams/slabs?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?------------------------------Muhannad Husain S.M.ASCEResearcherHohai UniversityNanjing+86 183 ------------------------------
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Post-tensioning concrete is the process of adding steel bars (tendons) below a mixture&#;s surface. Once the concrete fully cures, tension is applied to the steel bars. These reinforcements introduce stress to the concrete, which can prevent cracks and shifting down the road.
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