Cracked Versus Uncracked 1

[XR250]

I am still unclear on this issue. I am working on a metal stud job where I am using epoxy anchors in various situations. Many are for a cantilevered sill, so I have one bolt at each stud (16" O.C.) and it acts in tension only (about 1400 lbs ASD). This is an existing building with 9" PT slabs. They are going to x-ray it to make sure we don't hit anything.
If I assume cracked concrete, my anchor fails unless we go with a deeper embedment. If I assume un-cracked, it passes.
I have asked a few former collegues and they say they would assume un-cracked.
What are the rules?

 

[jdgengineer]

For seismic loads, ACI D.3.3.4.4 requires that you assume the concrete is cracked unless it can be demonstrated by calculation that the concrete remains uncracked. For non-seismic conditions I would probably consider anchors installed into the compression face as uncracked and into the tension face as cracked.

 

[XR250]

unless it can be demonstrated by calculation that the concrete remains uncracked

How is it even possible to show that? What type of cracking?

Seismic is minimal. Mostly wind. This is a long distance job so I have no idea of the tension or compression face. Cracked concrete really neuters these anchors!

 

[FoxSE14]

I second jdgengineer. My approach is: Compression zone = un-cracked. Tension zones = cracked. Seismic = cracked. Unsure = cracked. In a PT situation, you can likely make a better argument for un-cracked IN GENERAL, but I would ask the EOR (if you can).

 

[XR250]

It is a 30 year old building, so no original EOR to ask.
My feeling is if it is PT, I should be able to use un-cracked.

 

[FoxSE14]

How to demonstrate? Try qualitatively first.

Example--Assuming a normal DL + LL concrete beam, simple-span:
If you are anchoring at midspan to the underside , you are anchoring into the tension face, and I would design as 'cracked' every time because one would expect these fibers to always be in tension.

If you are, however, anchoring at the same mid-span location but into the top side of the beam, I would treat it as 'un-cracked' as one can assume the top fibers remain in compression under normal loading.

If you have a multi-span condition, you can expect tension zones at the top over supports (say span/3), and tension on bottom in middle third-ish (I am not accounting for Live-Load skipping, although you should).

For the PT case, using same DL+LL and simple-span beam as above: The pre-compression (P/A) provided by the post-tensioning may create a situation where, during service level loading, the expected tension stresses at the bottom of the beam may never/rarely be realized during service because P/A is large enough such that it overcomes any tension by My/I. This is an idealized scenario though. A cantilever, for example, may have different states of stress depending on backspan geometry, deflection requirements, tendon drape, amount of pre-compression etc.

 

[Structurecrg]

If this is in the top of a floor slab, is it near an area of negative moment? If not you may be ok. However in SDC C or higher you have to assume cracked concrete.

Separately, you mentioned your factored load in ASD.. I don't think you can use ASD loads with the strength reduction factors given in Appendix D. You may be even further from the mark.

 

[KootK]

I've seen several definitions of uncracked concrete that name shrinkage restraint as a source of fastener compromising cracks. With that on the table, even a PT slab might not qualify as uncracked locally. I wish that they'd ditch the untracked option altogether so that we could just all play by the same rules. I feel more comfortable with uncracked when there is some redundancy and, by virtue of probability, not all of the fasteners will land in cracks. You situation would qualify in my opinion.

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.

 

[XR250]

Separately, you mentioned your factored load in ASD.. I don't think you can use ASD loads with the strength reduction factors given in Appendix D. You may be even further from the mark.

My only load is wind so going back and forth with a factor of 1.6 should not be an issue. I do the stud design in ASD (because deflection controls 99% of my designs) and the anchors in LRFD.

For the PT case, using same DL+LL and simple-span beam as above: The pre-compression (P/A) provided by the post-tensioning may create a situation where, during service level loading, the expected tension stresses at the bottom of the beam may never/rarely be realized during service because P/A is large enough such that it overcomes any tension by My/I. This is an idealized scenario though. A cantilever, for example, may have different states of stress depending on backspan geometry, deflection requirements, etc.

I am only fastening into the slab and there is no cantilever. They really make this stuff complicated nowadays. I can't wait to retire!

Thanks for your advice

 

[XR250]

If this is in the top of a floor slab, is it near an area of negative moment? If not you may be ok. However in SDC C or higher you have to assume cracked concrete.

This is what I don't get. These bolts are not for major structural components - just some light exterior metal stud walls. An earthquake big enough to crack the slab will still have no effect on these walls as they weigh nothing. Yea, i can see if it was the Big Dig, but there is no safety issue here.

 

[FoxSE14]

I'm all for the 'keep it simple' route whenever possible. I think you've just justified using un-cracked!

 

[XR250]

I feel more comfortable with uncracked when there is some redundancy and, by virtue of probability, not all of the fasteners will land in cracks. You situation would qualify in my opinion.

That is what I was thinking. I agree, they should just ditch it or maybe add some stipulations about situations where you have redundancy, how critical the component is and the potential for life safety.

 

[Once20036]

Think about how/when the loads are applied. When you're dealing with the wind load, the slab is uncracked so the anchors are fine. When you're dealing with the seismic load, the slab is cracked but there is no significant force, so the anchors are fine.

 

[XR250]

Well said

 

[TLHS]

If shrinkage cracking were on the table, how could anyone reliably treat any concrete as uncracked? If that were the case, providing uncracked values would be straight up negligent.

 

[KootK]

I hear it. But from the perspective of the anchors, is a crack not a crack regardless of the cause? The clips below come from the following sources:

Link
Link

That's just what came up in a Google search. I don't claim any extraordinary expertise in this arena.

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.

 

[XR250]

Is this actually written by practicing engineers? Seems that everything is cracked because it is not possible to say it is not.

 

[KootK]

Is this actually written by practicing engineers?

Unlikely. The links that I provided will take you to the sources for the snippets.

Seems that everything is cracked because it is not possible to say it is not.

Yeah, that's the short of it.



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.

 

[XR250]

Do you still feel ok with using un-cracked in my situation?

 

[KootK]

Yessir, I do.

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.