Strand Elongation +10 % More Than Theoretical Elongation - Eng-Tips

Hi Rapt, Ingenuity, MacGruber22,

More info!

High elongations were encountered in many projects for both beams and slab. In some projects, it is in beams. In some, it is in slab.

Yes, my P is average force allowing for friction & wobble.

We had the strand tested locally for every project for breaking load, E values etc...

In here in Singapore, stressing operation is witnessed by Client Rep (R.E or R.T.O) & Main-con Rep. So my statement about over-stressing on site might be wrong.

I understand that short tendons give high elongation. Tendons are not short at least > 15 m.

RAPT,

Firstly, if my elongations assuming no friction & wobble are close to the actual site values, should I say it is satisfactory & in actual there was almost no friction?

If not, I will find the force where it goes non-linear from actual lab stress / strain curve. And I will calculate back the force from the actual site measurement: P after friction & wobble = Elongation*A*E/L. Use that force back to re-design & check the design.

One more thing, once our site team find out that the elongation is out of the tolerance, our site team hold the pressure 30 sec. If there was no further elongation due to holding pressure, Can I say that the strand didn't go into non-linear state.

What will be the impact of higher non-linear force on structure once it goes into non-linear? Only serviceability limit state & transfer stresses?

BTW once I key in the non-linear higher percentage of force, does RAPT software consider it accordingly?

According to our site engineer, they didn't experience any slippage at the onion dead-ends.

RAPT said: It does not take a lot of extra load to get a large increase in elongation as the strand goes non-linear at about 80-85%. Extension calculations normally assume linear behavior but if the strand starts going non-linear below the jacking load, this can be significant, especially for short tendons.

I think there is a high possibility for that to happen, since we are stressing total 79% ( 75% + 2 (jack losses) + 2% (seating losses)). A few more percent will cause the strand to go into non-linear.

Thanks for your replies.

Firstly, if my elongations assuming no friction & wobble are close to the actual site values, should I say it is satisfactory & in actual there was almost no friction?


Not necessarily. It is doubtful that there will be no friction. Even a perfect installation so that wobble is zero and with perfect steel and duct with no rust, you will still get friction from the curvature, I just wanted to know how high the figure is compared to no friction.

If not, I will find the force where it goes non-linear from actual lab stress / strain curve. And I will calculate back the force from the actual site measurement: P after friction & wobble = Elongation*A*E/L. Use that force back to re-design & check the design.

As the friction causes the force to reduce along the tendon, it would only be non-linear for a length near the stressing end. You would have to do it in short lengths with different effective E for each length. I would think it would only affect the fist few metres of the tendon.

One more thing, once our site team find out that the elongation is out of the tolerance, our site team hold the pressure 30 sec. If there was no further elongation due to holding pressure, Can I say that the strand didn't go into non-linear state.


Has nothing to do with it. This will happen immediately, not over a length of time.

What will be the impact of higher non-linear force on structure once it goes into non-linear? Only serviceability limit state & transfer stresses?

It is normally non-linear if the section is cracked at service anyway. RAPT would be allowing for that in the stress calculations for the tendons for strength and crack control and deflections. The problem would be if you overstressed too much so the initial condition is far too high a stress. It is not a sudden jump from elastic to plastic. There is a gradual change over about 10% of stressing range. If you look at the RAPT materials data for the strand it gives for a stress strain curve for the strand. You can adjust the parameters controlling this based on your strand tests.

BTW once I key in the non-linear higher percentage of force, does RAPT software consider it accordingly?

Not for extensions at the moment. Currently it assumes elastic. It is something I have been looking at allowing for.

According to our site engineer, they didn't experience any slippage at the onion dead-ends.

How could they possibly know that?