Strand Elongation +10 % More Than Theoretical Elongation 2

[struggle66]

Hi All,

Question Again

Lately I encountered a lot of high elongation dilemma 10% more than theoretical values about 15-20%. I really want to know why. Now I am trying to investigate all the possible causes one by one.

Elongation = PL/AE.

1. I've found out that physical properties of strands A & E values are almost the same as the values we used in our elongation calculations.

2. For the length of tendon L, we are using straight horizontal length from the start to the end of the tendon. Is it supposed to measure along the parabolic shape of the tendons which will be definitely longer than straight?

3. Our jacking force is 75% (Design Jacking Force) + 2% (Jack Losses) + 2% (Anchorages Losses or Seating Losses)= Total 79%.

And all our equipment are calibrated and I think over-stressing on site is also another factor.

Any comment on the possible cause or any other factor to consider?

Anyway it was already happened. Technically what can I do to find out or prove that there is no deficiency in structure and design intent was not affected. Calculate back the force P from the actual site measured elongation and re-check the design?

Thanks & have a nice day.

 

[pappyirl]

Have you included for the friction and wobble losses in the tendon? How is the dead end created? The unsheathed length of strand should be taken into account if a swaged end is not used.

 

[Ingenuity]

...and I think over-stressing on site is also another factor.

This is a bad site practice and may very well explain your above-calculated elongations.

 

[MacGruber22]

PL/AE is not a true elongation calculation for PT, as pappyirl mentions.

Is this a repair or new design? Is this a slab or beam?

And all our equipment are calibrated and I think over-stressing on site is also another factor.

You "think"? What did the jacking records indicate the jacking force to be?

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ingenuity]

Whilst friction and wobble effects need to be correctly accountered for, in the OP case of the measured elongation being greater than a simple PL/AE calc, a more correct friction and wobble will make the disparity worse.

Definitely need to check what your field crew are stressing to.

 

[MacGruber22]

Except that "L" used in the calcs appears to be the straight length along the span. "L" of cable is longer, and a lot more if the concrete section is deeper and the drape greater. Thus, friction/wobble aside, the actual elongations will be longer.

I suspect that this is a repair...

 

[Ingenuity]

I speculate it is NOT a repair. The OP is in Singapore and typically a bonded PT market.

Struggle66 - gives us some more details.

 

[MacGruber22]

I hear you, Ingenuity. Repair came to mind, because elongation is very uncertain in those situations.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ingenuity]

Repair came to mind, because elongation is very uncertain in those situations.

Totally agree, especially for those old, pesky, paper-wrapped unbonded tendons.

 

[Ingenuity]

Repair came to mind, because elongation is very uncertain in those situations.

Totally agree, especially with those old, pesky, paper-wrapped unbonded tendons.

 

[MacGruber22]

Ooooo...so good you responded twice!

"It is imperative Cunth doesn't get his hands on those codes."

 

[rapt]

Unless the L/D is relatively large, the difference in length between straight and draped tendons is not sufficient to worry about.

And Struggle knows what he is doing. PL/EA is the correct formula if P is the value allowing for friction and wobble.

Have you had the strand tested to see if the E value on the certificates is correct?

Are the tendons single or double end stressed and how long?

Have you calculated the extension assuming no friction and wobble to see what the maximum could possibly be? And have you checked the overall stress/straing response to make sure where it goes non-linear?

Where is the strand from? Is it coated in water soluble oil for transport (a lot of it is and this needs to be removed before it is installed and grout will not bond properly to it and onion dead ends tend to fail)?

Have you checked the stressing pump pressure they are using on site to make sure it is correct for the required load?

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 behaviour but if the strand starts going non-linear below the jacking load, this can be significant, especially for short tendons.

Are the tendons very short?

 

[struggle66]

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.

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.

 

[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?


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?

 

[AUCE98]

The tendon elongation is a measure of the average force in a tendon of a given length. The methods for determining losses due to friction and wobble are based on approximate equations with assumed values of friction coefficient and wobble coefficient. Accordingly, the computed average force along the tendon is merely a reasonable approximation of the actual values. The force imparted by a properly calibrated is a far more precise method of measuring the force on a tendon, with the measured elongations serving as a check on the force indicated by the jack. Additionally, because the tendon elongation is a measure of the average force along a tendon and not the maximum or minimum, over or under elongations do not imply that the tendon is overstressed or under stressed, respectfully. It simply implies that the average force along the tendon is higher or lower than the assumed value used in the elongation calculations.

Whenever deviations outside the 7% target range occur, there are several possibilities which may cause such differences. One obvious cause of differences between measured field elongations and calculated elongations is the tendon is not placed in accordance with the drawings from which the elongations were calculated. Deviations of tendon placement from the drawings can account for large variations between calculated and measured elongations.

When excessive elongation occurs, there are several possibilities which may cause such differences between the calculated and measured elongations. The jack calibration should be checked and the tendons should be inspected for signs of wire breakage. Normally, wire breakage will be apparent to the jack operator during the stressing procedure. Provided there is no tendon slippage a the dead end or anchorage movement at either end due to honeycomb in the concrete, the most probable cause for differences would be wobble and friction coefficients lower than the assumed values. For tendon elongations with excessive elongations which show no signs of anchorage movement or tendon damage or breakage, there is no rational reason to reject the tendon based on the strength and serviceability of the structure, as explained in the following paragraph.

Assuming the worst case scenario, which is the tendon is actually overstressed by some amount, the resulting condition can be analyzed in detail. Regarding the service load stresses in the concrete, additional tendon force would serve to increase the precompression in the structure, thereby decreasing the service level tensile stresses. The additional precompression would also be beneficial in regard to the strength aspect of the structure. However, the higher the service load tensile stresses in the tendon would mean that the prestressing strand would enter the yield plateau slightly earlier under factored loads.

The maximum strain in a prestressing strand is 0.00758 in/in (216/28,500). This maximum strain occurs at the jack only, and is reduced significantly upon release of the strand and the onset of seating loss. The minimum specified strain at failure for strand fabricated per ASTM A-416 is 3.5% or 0.035. The ratio of maximum strain at strand failure to the maximum strain at stressing is about 4.5.Accordingly, the strand provides a great deal of reserve strain capacity regarding any stress that may exist in a tendon for which excessive elongation develops during stressing.

Additionally, the failure mechanism for a member prestressed with an unbonded tendon is nearly always precipitated by failure of the concrete in the compressive zone, not a failure of the prestressing strand. In an unbonded member subjected to loadings far in excess of the service loads, several large cracks tend to develop at the locations of maximum moment. Since the tendon is not bonded to the concrete, the strain the steel is experiencing due to these cracks is distributed over the entire length of the tendon. However the concrete strains a these cracks are concentrated at these crack locations, causing a compression failure at these locations. Because the strains in the prestressing steel are distributed over the entire length of the tendon, in many cases, the maximum stress in the prestressing steel upon failure of an unbonded prestressed member is actually less than the maximum stress in the prestressing steel during stressing. In essence, the highest force the tendon will experience during the life of the structure is during stressing.

For tendons that are lower than the 7% target range, the same possibilities for differences between the measured and calculated elongations as mentioned above exist. Tendons not placed in accordance with the drawings as well as differences in the assumed and actual friction and wobble coefficients can cause large variations in the measured elongation. When deficient elongation occurs (elongations that are below the calculated value by more than the stated tolerance) there would be no concern about the stress levels in the tendons, provided the tendons were stressed by a properly calibrated jack. Additionally, the deficiency in tendon stress for an individual tendon may have a negligible impact on the concrete under service loads. If the average tendon elongation in a particular beam or grouping of prestressing strands is within the stated target range, no additional consideration should be taken. The deficiency in force of one individual tendon at a section may be offset by somewhat excessive force in other tendons at the same section. For these reasons tendons with lower elongation values rarely have an effect on the service load stress in the concrete. Accordingly, there would be no impact on the strength of the structure, since the tendon would still reach or exceed the maximum design force prior to reaching the structural capacity of the member.

 

[MacGruber22]

Good post, AUCE98!

"It is imperative Cunth doesn't get his hands on those codes."

 

[rapt]

AUC98

The maximum strain you have quoted is only correct if the jacking force is 216, modulus is 28,500 and the jacking force has stayed in the linear range (below about 65-70% UTS. Once the jacking force is over 85-88% the strain is very non-linear and will be much higher at jacking.

Relaxation losses are much higher at higher strains.

This is Bonded prestress. The strand can fail in tension very easily, especially if the stress in the strand is very high at jacking.

If the initial stress is very high, there is no "elastic" portion of the stress/strain curve to absorb extra strain as load is applied. The strand will be immediately in the plastic range. In this zone, it does not take a big increase in stress for the strain to reach the breaking strain as the plastic modulus is very low, more like 300 tan 28500 so strain increases very quickly.

Yes, it is possible to allow for this if it happens in a specific case. But not using normal design code calculations methods. Moment curvature calculations are necessary using the proper stress strain curves for the concrete and the steel. If the steel is in the plastic zone at service, this needs to be allowed for in deflection and crack control calculations. If the tendons are overstressed, extra transfer checks need to be made as well.

If the extensions are consistently 20% high, there is a definite need to find the reason. You do not want to be doing the calculations mentioned above for whole buildings.