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Tendon breakout force

Tendon breakout force

Tendon breakout force

(OP)

I want to determine the force to anchor the grout fill that will be used to close up inspection openings that were chipped into the beam soffits to permit examination of tendon condition. To do this "approximately", I reasoned that tendons don't generally break out of the soffit if they have 2" or more of cover, so I tried to estimate what the breakout force would be with 2" cover. This calculation is shown on the attached. I realize that this is very approximate.

Any comments?

I have a feeling that the calculated load may be too large (i.e. overly conservative).

Is the 1000 psi along the diagonal a reasonable failure stress, or is it too high?

Is the 12" length of breakout reasonable?

Are there any other suggestions of how to estimate the breakout force?

RE: Tendon breakout force

How is the force getting into the tendon to provide an outward component?

RE: Tendon breakout force

There shouldn't be any breakout force in the tendon if the inspection port is placed in a positive moment region, as is usually the case. The tendon is pushing in the opposite direction against undisturbed concrete.

BA

RE: Tendon breakout force

(OP)
To BAretired (and dik): yes the opening is in the positive moment region near midspan, as you say. Yes the stressed tendon is pushing away from the bottom surface as you say, but this is only while the tendon is stressed; when the last of the wires break due to corrosion, at that instant the tendon will no longer be stressed and will move in the opposite direction (the end will retract away from the break location) when the last 3 wires of the 7-wire strand break. At that instant the tendon breaks out thru the bottom.

Last week one of the tendons failed in this manner and knocked the metal cover plate that was fastened over the opening. Not the first time this has happened. The usual breakage mechanism for the 7-wire tendon is that wire #1 breaks, then some years later wire #2 breaks and so on until wire #4 breaks. At that time there are only 3 wires left, and they immediately break because the stress in the 3 wires exceed the ultimate tensile strength of the 7-wire strand.

RE: Tendon breakout force

Thanks... hadn't considered that condition... don't think I ever have...

Added: Just realised that if a strand failed, breakout would be one of my least concerns...

Dik

RE: Tendon breakout force

I wasn't thinking about that either, but I do recall evidence in a hearing some years ago in which an unbonded tendon reportedly broke through the top of a floor slab of an apartment building, presumably in a negative moment region. A lady had been exercising in front of her TV; fortunately, she left the spot for a few minutes during which time the tendon lashed out precisely where she had been sitting. It was proposed to correct the problem by applying 1/8" thick steel armor plate over the entire floor. I do not know if that work was carried out or if such measures would be effective in the event of a similar tendon breakout.


BA

RE: Tendon breakout force

(OP)
To BARetired: Right on.

RE: Tendon breakout force

ajk1,

There would be a "whipping" force when the strand releases and makes contact with the cover concrete plate or cover concrete, but nothing like the force you have calculated. I am not sure how you would calculate it, but I do not believe it would be sufficient to break through 2" of cover concrete.

Another good reason to use bonded PT. As long as it is grouted properly we would never have this sort of problem.

In the one case I have seen in 40 years where tendons were not grouted and some strands snapped after about 10 years, there were no problems like this. Cover was only 1", but then the .4mm metal duct would have increased the breakout capacity.

RE: Tendon breakout force

Quote (ajk1)

Are there any other suggestions of how to estimate the breakout force?

You have a breakout force of 54 kips, so I assume you just multiplied the area of a 0.6" dia strand (0.215 in2) and 250 ksi MUTS.

The approach of calculating a static force and assuming it is equivalent to a dynamic one is going to be riddled with 'errors'.

You may wish to look at it from a elastic strain energy approach. Calculate the elastic energy (you have A,E, P, assume a length of where the strand is severed, L, calculate Ue = P2L/2EA) and that will be the impact energy that get imparted on the cover concrete. Assume the elastic strain energy is transferred to the cover concrete by Ut = F2/2k, where k will be the elastic stiffness, k, of the cover concrete (not an easy thing to do), F can be then calculated which will be the impact force. Maybe best to do an iterative solution, assume a k and L value, calc F, vary k, vary L, see effect on F.

BUT...you have to account for energy losses due to dynamic friction due to wobble and curvature of the tendon (based upon 'real value' of the old grease/sheath system), how the individual strand is bundled within the tendon group, how to account for the impact force not being perpendicular to the cover concrete, and this is going to get complicated real fast.

A field-based or lab-based measurement of the 'breakout/impact force' will be more practical than a theoretical solution, given the number of variables/assumptions that have to be made. Maybe you could convince your client to spend a few $ on such an investigation on a real tendon on this project.

I recall that on one of your past related threads the structure was monitored by an acoustic monitoring system. If this structure is the same structure, then when one wire breaks doesn't the monitoring system pick it up, such that you can then be on alert or do a preventive measure to this tendon group and its associated inspection locations?

Quote (ajk1)

Is the 1000 psi along the diagonal a reasonable failure stress, or is it too high?

1000 psi for a patch is going to be optimistic. I think for a trowel applied patch you will be closer to 200-400 psi range - based upon work that I have been involved with on field bond test of overhead concrete repairs.

If the repair technique used SCC with 'form and pump' then 400+ psi may be more appropriate.

If you think in terms of tensile strength of concrete via modulus of rupture, a value of 7.5√f'c would be about 530 psi for 5000 psi compressive strength. Just a value for comparison purposes.


RE: Tendon breakout force

(OP)

To Ingenuity - Thanks for your detailed response. Very much appreciated. You suggest an energy method. That in fact was the method I tried before resorting to the concrete breakout strength method that I posted here. The energy method gave even greater force than the concrete breakout method! But I may have not done the energy method correctly (been many many decades since I studied that in school) , and I realize that the energy method did not account for items like friction on the failing tendon. I will study the energy procedure that you suggest. Thank you for that.

The energy method did give reasonable answers when I was looking at the option of closing the opening with a 16 gauge steel plate, but that is much more flexible than the grout closure which ends up being 5"± thick.

I realize that the bond strength of repair materials is in the range that you suggest, but that is a different effect (direct tension) than the breakout along the diagonal failure surface shown in my sketch. But you may be right that the 1000 psi may be too high. It does seem perhaps to be more like a "slant shear" strength. I wonder If I should look into what the slant shear strength of concrete is. I am trying to establish what force I have to design the dowels connecting the 5" thick grout closure to the beam soffit.

Yes, as you say, this structure does have an acoustic monitoring system but that system cannot distinguish between a single wire failure and multiple wire failures. All it can identify is if a wire fails. Also that system depends on human interpretation of the signals, which is subject to error, but it is the best we can do.

On another matter, often I type a lot and then it all disappears, and I have lost it. I don't know why this happens, but I believe there is a way of getting it back on the screen, but I forget how. Do you or anyone know how that is done? I think have to press control and another key simultaneously.

RE: Tendon breakout force

(OP)
To Ingenuity: one further comment - the 54 kips is not derived from the strength of the tendon. It is arrived at from my attempt to evaluate the breakout strength along the failure surface shown in the skecth that was attached in my post. The calculation is also on that diagram.

RE: Tendon breakout force

Thanks for the added info... Dik

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