post tensioning calculations 1

[boffintech]

Considering post-tensioning: specs call for tendons to be stressed within 7% of calculated elongation.

A calculated elongation of 2 3/4” results in a possible 7% variance of 0.1925”.

The rub: The Field Procedures Manual for Unbonded Single Strand Tendons requires that tendon elongations be measured to the nearest 1/8”. If during stressing a tendon is slightly more than 3/16” (0.1875) over the 2 3/4” it is recorded as 3”.

Since tendon measurement is rounded off to the nearest 1/8” a calculated elongation of 2 3/4” has a variance of 1/4” and a possible low of 2 1/2” and high of 3” results.

But is it really a 1/4”? If the actual elongation is measured at 3” this is 9.1% over the calculated elongation. ((2 3/4 - 3)/2 3/4) x 100 = 9.1%

The question: The calculated elongation is 2 3/4”. 7% of 2 3/4” is 0.1925. Does the 0.1925 round off to 1/8” or 1/4” to determine the low/high variance? The 7% variance of 0.1925 is closer to 1/4” than 1/8” but if 1/4” is used it technically results in a 9.1% tolerance.

Any ideas?

 

[rapt]

Savage54

Personally I worked for a speciality PT company for 12 years and was often involved in site works. A lot of inspection and even did some stressing and grouting myself. I know that Ingenuity has been involved in PT site works for at least the last 15 years and has a lot more experience with PT installation, stressing and repairs than most seeing he owns his own PT company and repair company (repairing PT buildings stuffed up by unskilled designers and installers).

Obviously your definition of Skilled is different to ours. I think I made a comment in an earlier response above on this. Something about the difference between what does happen and what should happen regarding the quality and skills of the PT workforce betweem Mexico and Alaska. And you can believe what you like, it is a free country.

The PT supplier, installer and stresser have entered into a contract to supply a prestress system and perform a prestress installation to a certain standard. Involved in this is achieving tendon extensions.
If the PT people do not think they can achieve them they should not be doing the work. If the problem is an isolated case then this can be worked around. If the problem is systemic, then work practices etc need to be looked at.
If the engineer has no experience, it should go to a senior with experience or they should not be doing the work either.

 

[SAVAGE54]

Rapt
I totally agree with your comments. These people "engineers" that responded to the emails are try to measure elongations to the nearest 1/64th inch. It is impossilbe. They where saying since there are out of tolerence 7.111% then something was wrong. Contractor ERROR

By the way the senior engineer your are speaking of is me.

It is only recently that the PTI started requiring some basic training for PT inspectors and jack operators.

Most the time the calibration for the jack is not on site.

I am not putting down the labor force conducting the operations. Heaven knows they are making more for installing it the I did designing it. It is engineers need to understand thier are a lot of assumations with tensioning. These assumptions become big part of the errors found in short elongations. For longer elongations the assumations do not effect the results as much.