Post Tensioning Elongation

[ztengguy]

We have been asked to provide PT elongation calcs. I cant seem to find a standard equation to use, is that something the mfg of the PT cable would provide for us?

 

[BAretired]

He would provide a value of Modulus of Elasticity for the strand you are using. It has been a while since I did that, but I recall values between 28e6 and 29e6 psi for PT strand.

BA

 

[ash060]

It is roughly delta = PL/AE. It will vary depending on friction and the cable

 

[AELLC]

Here's an Excel 2007 (.xlsx) for tendon elongation - for 1/2" diameter tendons.

0.076" per foot.

 

[yakpol]

Check out demo spreadsheet at

http://www.yakpol.net/PSLoss.html

It will calculate elongations accounting for friction losses. Also find formulas on the page "Theory".

 

[ztengguy]

hey, thanks guys, I will look in to them. I think that elongations should be measured first, not put on the drawings so the PT guy thinks he has to match them.

 

[BAretired]

The PT guys have two ways to measure cable tension, (1) direct measurement of force and (2) elongation. The EOR normally specifies the initial tension required and leaves the measurement to the technicians in the field. They keep thorough notes showing both the gauge reading and the elongation. If there is a discrepancy between the two methods, further study is needed to find out why.

BA

 

[AELLC]

My post above related to post-tension slab on grade, not elevated slabs. We always indicate elongations on the drawings.

 

[ztengguy]

ours is a bit wacky...its post tensioning about 8 culverts together...so they will pull the joint together, then they need to start measuring the elongation. No real structural worthwhile, the DOT just wants them pulled together.

 

[rapt]

ztengguy,

Any decent text book on PT and many design codes have formulae for calculating the Prestressing force in tendons, allowing for friction and elastic shortening.

Then use PL/EA but remember P varies along the tendon, it is not the jacking force.

Do not believe ext/length numbers. They vary depending on the situation.

I agree that expected extensions should never be made known to the stressers, but that is not the PTI practice who recommend that they be on the drawings. I have seen too many cases of the expected results being achieved (according to the records) to recommend that practice. Their arguement tends to be that knowing the extension will avoid overstressing. Watching the gauge will avoid overstressing! Stressing to an extension can often cause overstressing if there is extra friction or a blockage in the tendon.