Post-tensioned tendons jacked beyond yield

[ctb74]

We have a situation where the contractor has screwed up during the stressing operation and instead of stressing the 1/2" diameter 270 ksi ultimate strands to 0.8 of fpu (216 ksi or 33kips) they have stressed many of the tendons beyond theoretical yield of 0.9fpu and in one or two cases beyond fpu (without the tendon breaking). The contractor wants to leave the overstressed tendons alone. The only thing we have found on the subject is from page 27 of the document below (State of CA Prestress Manual) related to bridges.

http://www.dot.ca.gov/hq/esc/construction/manuals/OSCCompleteManuals/PrestressManual_Rev1.pdf

Anyone have advise or suggestions?

 

[Zambo]

If these tendons have really been stressed to up to, and in some cases exceeding, 100% of UTS then of course they are not acceptable. Whatever code you are working to should be followed and that will include the stress at service loads.

But have the stressing calculations been correctly carried out including allowance for friction losses and at lock-off. The stressing records must look very unusual when reviewing the tendon extensions against force when the yield strength is exceeded

 

[strguy11]

I agree with Zambo. What were the actual elongations compared to the theoretical elongations?

Have you checked to see what the actual area of the strand/tendon was? Usually this vary a little. If the actual strand/tendon area was more than the theoretical tendon area then the actual applied stress would be less. Just a thought for you to look at. Also the modulus of Elasticity may vary as well. Get these values from the manufacturer before doing anything.

You still may need to detension the stands and remove and replace, but these are an option for you.

 

[Zambo]

strguy11, yes agreed he should get the mill cert for the strand used and then work from there.

ctb74, what data do you have in the stressing reports? Do you have the jacking force and tendon elongation at intermediate as well as final jacking force?

Has the jack got a valid calibration certificate?

 

[dik]

Can anyone suggest, other than document compliance, why the tendons should not function as intended? As long as they didn't break... when relaxed to design levels, they should behave 'elastically'.

Dik

 

[Zambo]

The threadstarter has not responded, so we do not have too much informatiohn. But as it is we do not know how the service loads compare to the yield stress of the tendons.

 

[dik]

Zambo... does that have any real bearing. I would think that once they had been overstressed and installed at the original design value that things would be fine.

Only changes:
1.) You have confirmed that they can achieve the load.
2.) They will behave in a more predictable 'elastic' fashion.

Dik

 

[Zambo]

dik,

why do you say they have been "installed at the original design value" my understanding from the first post is that they have been overstressed in some cases to over 100% of theoretical UTS and then locked off. From the post we don't know the final condition of the tendons. I assume they are still overstressed according to the code, but unless some action was taken (which the post suggests not) presumambly the tendons are still very much overstressed, perhaps 90% or more of UTS.

In a case like this I agree the tendons don't necessarily need to be rejected just because 80% of UTS (or whethever the local code says) has been exceeded, but wouldn't the designer want to check that his design is satisfied under service loads.

 

[dik]

my misunderstanding... I thought that they had been relaxed to the design values... They should be relaxed... if the end hasn't already been cut off...

I thought that the problem had been corrected and that the strand had only been overstressed prior to final condition.

Dik

 

[rapt]

If the stress in the tendons has been maintained, there are lots of problems.

- Relaxation of the steel is far higher. I have not seen tests that show the numbers if stress to near UTS.
- if the steel is in the plastic zone, small increases in loading will lead to large increases in strain and premature failure of the member. It will never achieve its ultimate capacity and the strand could easily fail below the service loading depending on the overall design.

Basically, it would be hard to justify accepting the member based on ultimate strength conditions if this was the case.