Post-tension tensioning irregularities 1

[hardhatpoke]

I am working on a project and doing the elongation reports for a large apartment complex subdivision. All the building's tendons are tensioning fine at 5500 psi lengths from 14 to 100+ feet but when the garages are tensioned specifically the 23 foot long tendons they are needing nearly 6000 psi to get to the specified elongation. The long tendons on the garages (76 foot) are tensoning fine at 5500 psi on the same garage. Why are these short tendons only on the garages needing so much more pull? all the concrete is same mix same day, same jack pulling tendons.

 

[kelowna]

Could it be a calculation error?

 

[kelowna]

Talk with the designer

 

[hokie66]

Friction? I am thinking that the 23' tendons are in a single span element, thus are on the same side of the duct full length. The longer ones are multiple spans, so the friction forces vary along the length.

 

[hardhatpoke]

the elongations values are in line with the other short tendons in the building 1 3/4 inch +/- 10% (which pull fine) for instance a 27 foot tendon in the building needs to be pulled 2 inches. I don't know about friction except that the tendons of similar length on the buildings pull fine at 5500 psi. I may call the design engineer and run it by him.

 

[rapt]

hokie,

If the calculated extensions were calculated correctly, the different amount of angle change and friction should have been allowed for in the estimated extensions.

hardhatpoke,

Stressing should be done to load/pressure with checks on extension after the stressing is finished to make sure all is ok. It should not be done to an extension. This is a recipe for over stressing and breaking strands. The pressure in the gauge (if it is calibrated correctly) is telling you the force in the strand at the stressing end. The maximum allowed value of this force is an important limit and it is normally the defined stressing force, so overstressing to achieve a calculated extension results in higher forces in the strand than allowed and the possibility that the strand is past yield during the stressing operation. If there were a problem in the duct, such as a slurry blockage, you might only be stressing half the length of the strand and you would end up snapping the strand! This is BAD practice.

We often find that measured extensions on short tendons are high. Never have been able to come up with a definitive reason. If yours are low, you might need to look at the calculated values. Are thhese bonded or unbonded tendons?

 

[hardhatpoke]

these are unbonded tendons

 

[hardhatpoke]

The machine is calibrated correctly and pulling all other tenons on site well and the calculations check (elong = P * L / (E * A)) with other factors (Loss of prestress due to friction, elastic shortening, creep and shrinkage of concrete, and steel tendon relaxation)taken in to account simplifies to basically 0.078 in/ft of tendon which is 1 3/4 inch for a 23 foot tendon.

 

[hardhatpoke]

at the calibrated psi we can only seem to get 1 1/4 to 1 1/2 inch of elongation we need another 1/2" to 1/8" to get to the 10% min of 1 5/8". So basiclly I should be pulling the the calibrated psi even if they are short.

 

[Zambo]

Have you tried checking the actual length of the tendons instead of relying on drawings and sressing data sheets. It may be that there has been a change to the length on site which hasn't been updated on your drawings.

I agree with RAPT that stressing should be done to force with elongations used as a check. In this case the check shows a problem which should be investigated.

 

[hardhatpoke]

I have not checked lenghts I will do that and The design engineer has said to "It is to be based on elongations – tendons pulled to the specified elongations." But thanks I will look at the lenghts

 

[strguy11]

Have you checked the actual area of the tendons vs. the design area?

 

[hardhatpoke]

Zambo you win the tendons when measured in the field were 2 foot shorter than plans. Thanks all for the input.

 

[hokie66]

The simplest answer is often the best. Good thinking, Zambo.

 

[rapt]

hardhatpoke,

A very good reason for stressing to pressure and NOT stressing to extension. Those strands are now overstressed by nearly 10%!

What are you going to do to solve that problem?

 

[amsysco]

It seems like this issue has been solved, but another possible cause could be excessive "seating" loss (a function of the wedges grabbing strand after stressing jack is released). Typically, you can expect 1/4" of seating loss. However, if you have 3/8"-1/2" due to a dirty anchor cavity, then your measured elongation would on the low side.

With a shorter length tendon, 1/8" is a larger percentage of the expected elongation. Sometimes, it may be easier to replace these with conventional rebar.