Concrete breakout due to Pensioning of anchor boly 1

[allimuthug]

Hi Everybody

The Picture attached shows a post installed anchor bolt being pretension-ed.
Case a - Concrete breakout takes place under the anchor bolt due to pretensioning.
Case b - Concrete breakout doesnt occur since the nuts are in compression and the bolt is in tension.

Can you please explain me which is the best possible case as I am confused.

 

[WARose]

The "best" case is "case b".....but that isn't how it is going to behave. It's going to behave like the diagram on the left. Every anchor I've ever tensioned like this.....I've checked as per Appendix D of ACI 318.

By the way, this doesn't look like a "post installed" anchor to me.......more like a cast-in-place anchor with a nut and washer.

 

[KootK]

Huh. I actually vote the other way. I think that case B is much nearer the truth and consistent with typical practice for prestressed anchor bolts. You may still have to deal with bursting / side face blowout of course.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DETstru]

There can't be breakout.

Imagine if all the concrete below the embedded nut was gone. So you just had a chunk of concrete between the embedded nut and the upper nut.

So if you pulled on the bolt (upward) from above the surface, the whole thing would just lift. Sort of like a "breakout."

But, if all you did was tension the nut, it would simply squeeze the chunk of concrete and nothing would move. The system remains static. No breakout.

 

[WARose]

[blue](KootK)[/blue]

I think that case B is much nearer the truth and consistent with typical practice for prestressed anchor bolts.

A case can be made for it......but there are too many "what ifs" in that to be 100% sure. I spoke with a anchor manufacturer (i.e. headed ones like this) on this subject some years back and he specifically told me that testing they had done had resulted in cracking and/or failure in certain cases. (Especially in cases when: the embedded plate was significantly large (and stiff), the pretension load was very large, you were anchoring in a zone with very little reinforcement, the bolt was small enough, etc, etc. Or a combination thereof.)

As a result, to be on the safe side, I have (since then) taken the breakout approach.

 

[canwesteng]

If there are shims or other such uneven load distribution between the base plate and the concrete, I think it could break out. Otherwise I think the cracking you would see would be caused by side face blowout.

 

[KootK]

Thanks for expanding upon your opinion WARose. I tend to defer to testing, even anecdotally reported testing, for most things. Would it be correct to assume that the breakout load associated with prestressing would not be additive to any externally applied tensions? That's how I'm seeing it.

The sketch below shows how I'd sort of see Poisson's ratio leading to a fracture in spite of the fact that there's no net external load. Interestingly, there's another active thread here right now that's related: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

[blue](KootK)[/blue]

Would it be correct to assume that the breakout load associated with prestressing would not be additive to any externally applied tensions? That's how I'm seeing it.

That is how I see it too.

[blue](KootK)[/blue]


The sketch below shows how I'd sort of see Poisson's ratio leading to a fracture in spite of the fact that there's no net external load.

Good sketch. It is analogous to bursting forces from anchorage of prestressed tendons.....only difference is: the "edge" in the compression direction is mighty close. There is no reason to believe all the compression forces from the end nut/plate are going to run straight to that bolt head. (It's not "smart" concrete.)

I think stiffness is playing a role here as well (as always). Referencing the testing I mentioned before, the guy mentioned that the break out "cone" (or cracks) in some cases was much smaller (in radius) than it should have been at the surface. I think what is happening there is a concrete "sleeve" (around the bolt) forming when it gets tightened. (Analogous to a bolt in a canister.) Under certain circumstances, the "sleeve" is stiff enough to attract force it can't take. (And not ductile enough to avoid breakout by giving a bit and making the concrete directly under the head take it all.)

 

[Once20036]

I`m with DETstru on this one. If you were to get a failure across the cone illustrated you'd just be able to lift the intact bolt and cone of concrete out of the larger block and walk away with it. That just doesn't make sense. You can tighten a nut such that the nut and bolt do not fail, but the whole assembly just lifts out?!
Has anyone ever seen a failure of this type?

 

[kingnero]

I'm not sure I fully agree with the above sketch - see my idea in attached jpg.
I don't believe a nice cone-shaped chunk of concrete would come loose after overtightening a similar set-up.
I'd buy that cracks would start to run first in a "horizontal" plane, then either 45° upwards or downwards, initiating from the bottom washer/plate.

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[KootK]

I neglected to show something somewhat important in my sketch. There should also be some high, local tension forces in the concrete across the failure cone near to where the crack tips open. Essentially, you get a crack chasing it's way up through a brittle material. Without those tension forces, my sketch would not quite be in equilibrium.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.