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RE: Problem with P/T Tendon Stressing Procedure[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Problem with P/T Tendon Stressing Procedure
- From: Gil Brock <gil(--nospam--at)raptsoftware.com>
- Date: Mon, 23 Oct 2006 08:28:08 +1000
The simple answer is that the extensions should not be listed on the shop drawings. We never allow that and it is the reason I was saying I have a problem with the PTI procedures. Put them on a separate sheet of paper that stays in the office. I know a good stresser will have an idea of the expected extension from the length of the tendon but there is enough variation in this (10 - 20%) to stop them from fudging results. It just lets them know if there is a real problem.
Your 180' long tendons must have been perfectly straight, supported on a rebar mat, un-profiled.and well greased to achieve those figures. The procedure is acceptable.
At 07:44 AM 21/10/2006, you wrote:
I totally agree with Gil on the stressing contractor knowing what they are chasing for. All they are looking for is a +/- 7% * theoritical elongation and it is so very easy for them to fudge it. I don't however know what the best way is for the engineers to conceal the information. When I review the shop drawings, I spot check the theoritical elongation based on 0.0079 in/ foot length of the cable (PL/AE). If that is not present, then 2 sets of drawings (for review and for field) would have to be created and I don't know how the contractor would fit it in his well tailored PROFIT MARGINS.
I have had jobs where the stressing contractor has stressed cables longer than 180 ft from one side to the specified gage pressure (a force of about 33000 lb) and then gone back to the other side and stressed to the same gage pressure and more often than not, only made up the seating loss. I don't know if thats an acceptable procedure. They have been typically achieving 90-95% of the theoritical elongation from stressing on one side. The remaining 5% were made up from stressing the other end.
"Stuart, Matthew" <mstuart(--nospam--at)schoordepalma.com> wrote:
- You really need to do a wedge set, friction loss calc (including wobble effects) to find out what the actual stresses are from end to end. The P/T Manual has examples of how to do this by hand, ADAPT also has software that will do the same. The P/T vendor for the job should also be able to do a run (or at least have the engineer he used to come up with the required number of tendons based on the minimum force and drape requirements specified in your CD's). If all of the above fails, send me a diagram of the drape, and other project specific particulars and I'll run an analysis using my own software.
- Matthew Stuart
- From: Gil Brock [ mailto:gil(--nospam--at)raptsoftware.com]
- Sent: Wed 10/18/2006 7:14 PM
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Problem with P/T Tendon Stressing Procedure
- 1 With this procedure they should not have achieved the target extensions. I would want to know how they got the numbers right by only applying half the prestress at one end and the full prestress at the other. One problem I have with the PTI prestressing rules is that the stressers actually know what extension result they are chasing. I know of several cases where stressers have fudged the results to agree with what they were chasing. In our practice, they are never given copies of the expected extensions.
- 2 If they only stressed to half jacking force at the first end, then the tendons forces at that end and for the first 1/2 of the tendon are well below what is required by your design. And your slab is under strength. It is not only a seating problem, the prestress force in the tendons is deficient and the service and ultimate strength are deficient. Yes I would definitely get them to stress the first end to the full jacking force.
- At 07:21 AM 19/10/2006, you wrote:
- Hi List,
- I just received a tendon stressing report for a small elevated flat plate podium slab. When I opened the file, a glaring stressing procedure error was readily apparent. I would appreciate comments from list members regarding the situation and my opinion.
- 46 tendons in the banded direction are on the order of 140-160 ft long. The design drawings and the field placement of the tendons provide for stressing these at each end. The stressing record indicates approximately equal elongation pulled from each end of each banded tendon. At this point I contacted the jobsite to verify my observation that the tendons must not have been fully tensioned to 33 kips at the first end. The inspector confirmed that first end of the tendons were only stressed to half of the specified 33 kips. My concern is that this procedure results in an unknown quality of wedge seating and hence unknown quality of gripping force at this end of the tendon creating some possibility of future tendon slippage and force loss. Although the total target elongations were achieved by the procedure used, I am currently planning to ask them to put the ram back on the tendons at the first stressing end and take it up to the full force. Deal or No Deal?
- Thanks in advance for commentary offered.
- Brent Koch
- Koch Engineering, Inc.
- Regards Gil Brock
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