Kinked 1/2" Diameter, 7-Wire Prestressing Strand - Acceptability Parameters and Causes 1

[KootK]

My favorite precaster has some strand that has become permanently kinked in the unloaded state. Their speculative explanation for the cause is that the strand has been pulled off of the spool and through the pulling head at too sharp of an angle. The questions that we're trying to answer are these:

1) Are there additional explanations that should be considered?

2) What are the limits of acceptability on this? Does this section of strand need to be scrapped or can it be pulled taught and used?

I don't know the answer to this which is why I've created this thread and have reached out to PCI. I do have the following thoughts, however:

3) Even with some locked in, inelastic plastic stresses, I suspect that the strand could probably just "pull through" to the normally assumed, full tension stress state given that the locked in bending is internally self-equilibrating.

4) I know that prestressing strand is less ductile than say, mild rebar, and does need to be babied a bit more to prevent fracture. I worry somewhat that, even with the pull through mechanism of #3, the individual wires of the strand may be at substantially different tensile strains when that uniform stress is reached and that may result in premature tensile fracture of some strands.

5) I worry a bit that the winding of the strands may have come apart somewhat during the bending process such that, when they come back together, perhaps there will be detrimental stress concentrations etc.

6) When prestressing strand is intentionally deviated / harped, the curvature induced in the strand would typically be considerably more than the photo shows. That’s encouraging although the difference may be the more controlled environment under which the deviation is applied.

So what do we think, can strand bent like this still be used?

 

[miningman]

Depends very much on how critical future lifts are. Are you willing to incur the risk of a failure. For hoist ropes within the mining industry , by law such a cable would be scrapped immediately.

 

[KootK]

Thanks for your response miningman. This isn't for a lift however. The strand would be incorporated into a precast element (plank, beam, wall panel). In many cases that element would be critical but the individual strand would be less so. The offending strand would be one of between five and twenty in the element being fabricated.

This reminds me, however, that PCI actually has a method for the design of lifting loops for precast members that basically has you bend strand to a tight radius and embed it. That being the case, I imagine that the factors of safety and the working stress levels are quite different.

 

[rapt]

I do not have research on it, but if it was permanently deformed like that, that strand would be in the bin if I was involved!

 

[dhengr]

KootK:
1) They dragged/pulled that over something in the process, and the left side of the cable strand looks a little flattened (abraded?) in the process, and I would worry about that. RE: their explanation, what the heck, was the spool in the next room and around a few corners, or what? Many times a kink condition comes with a twisted wire rope problem during the unspooling. Keeping some tension on the wire rope during unspooling is important, a brake of some sort on the spool, so it doesn’t keep spinning. Can that section be cut out and still leave a useful piece of cable strand? Is saving 100’ of cable worth the potential risks/liability, if there is still something useful left, like lifting loops, shorter stressing bed, such like. I would look very carefully at the cable right in the region of the bend, for any damage. You’ve already hit on most of the conditions which I would think could be could be a problem. I’m assuming they want some help explaining what happened, and what the potential consequences are, with some discussion of the potential defects/deficiencies in the existing cable; not a rock-solid guarantee from you.

2) PCI should have some Q.C. thoughts on the matter, otherwise your guess is as good as mine. Their wire rope supplier might be the go-to people on this matter. What will they tolerate and guarantee under these conditions? There must be some minor limit which would normally be tolerated. I agree that all else being o.k., some minor yielding should not be a problem. They would just pull through/past this on the stress/strain curve. But, ‘all else being o.k.’ is the big question. Look into the effects of cold working, strain aging and Bauschinger effects. Yield and tensile strength might increase a little and ductility will decrease as you get into the strain hardening region of the stress/strain curve.

3 & 4) I agree, except you may not be able to really see the one bad/damaged wire in your close examination, ductility will likely be reduced, and any minor wire dent/nick becomes a big stress raiser. In #4, I’d call your “uniform stress,” the ‘final stress,’ since the non-uniform (residual stresses, plastic deformations) are part of what’s worrying us.

5 & 6) I agree. I have the same thoughts and concerns as you do.

Can they position this strand in the member length such that this stand (bent/strained) location would be in the lowest stresses portion of the member, as apposed to the max. moment location or right at the stressing end? This is starting to really reach for straws, to save a little wire rope. This is one of 15 other strands in the member does this relieve our heartburn any? It might be interesting to cut this section out, and test it to see what we could learn to answer some of these questions.

 

[JLNJ]

You are in good company KootK.

Roebling had similar problems with the cables on the Brooklyn Bridge and he had to make the call on the fly. Suffering at the time from the bends, no less.

 

[Ingenuity]

Their speculative explanation for the cause is that the strand has been pulled off of the spool and through the pulling head at too sharp of an angle

Very plausible - because I have done exactly that a few times.

Bare 1/2" dia strand comes in 12,000 ft reel-LESS coils. It is placed in a bri-pak for easy distribution in the field/precast facility. You pull the strand from the center of the coil - from its inside diameter. Depending on the type of bri-pak used, when you get close to the end of the coil (say last 20%) the coil no longer stays in a nice patterned formation - if a few circumferences of the strand 'fall' away from the patterned formation is sort of gets knotted/restricted from free-flowing. Add in the torsional effect of a coiled strand wishing to straighten as it is pulled, so as you get towards the end of the coil there is a bit of violent 'whipping' and if there is a resulting tangle within the coil and if the operator keeps pulling then kinks can form.

Sometimes a figure-8 is formed where the strand loops and twists. Got to be careful un-doing a figure-8 - it can whip back and smack you in the head or pinch a few fingers. You know how I know? Because I have been a victim!

For less than $1 per foot (or what ever number of Loonies that is in CAN) best to trash the kinked segments/s of strand.

 

[Ron]

Deformation has already occurred. This is high strength stuff and when it gets stressed for placement, it will be "redeformed" by straightening. This will likely cause some strain hardening, making the tendon more susceptible to stress corrosion cracking. Scrap it.

 

[Craig_H]

From my comfy armchair, the photo of the kink looks suspiciously flat on the left side. Putting aside the relative level of trust between you and the precaster, I'd be willing to wager that the wire may have been crushed by something. As each strand is deformed wire, and essentially a solid element instead of individual braids of small wire, it's very difficult to kink that stuff. I'd be happy to be proven wrong, but my memory of unwinding 18 mm diameter strand wire is not a fond one. That stuff is so stiff that it'll break bones it decides that it wants to straighten out while bent.

 

[r13]

When there is permanent deformation, the strands have been bend beyond yield then recovered some, the reload strength is un-predicable. Why risk?

Note, I think the permanent deformation was caused by the persistent pulling over a jammed device. If the pull force is T, and it makes 30° from the vertical, the localized resultant force around the device can approach 1.25T or higher. This stress concentration is part of the reason for very high safety factor requirement for lifting device.