ACI Appendix D - Concrete failure area for a group of anchors 1

[Deener]

I'm using Appendix D to check the resistance of a group of anchors embedded into a 24" thick, 10 ksi slab. Accounting for overlap effects of the individual anchor failure cones, I still get a very large failure area (approximately 6500 in^2). This got me to thinking that when that failure area becomes quite large, it may not be the governing failure. I would think that it's possible for the slab to fail in shear before this area fails. This shear failure would have to reference an area that is the perimeter of the anchor group multiplied by the depth of the slab (also accounting for reinforcement if present). I don't see a mention of this failure mechanism in Annex D. Am I missing something? I'm thinking that ACI section 11.12 (Shear and Torsion - special provisions for slabs and footings) should be referenced in appendix D.
Thanks in advance
P.S. - this is my first post so I apologize if I have posted in the wrong forum

 

[Bagman2524]

You've got the correct forum.
That would be a punching shear failure and yes it needs to be checked. That would be in a separate section of ACI-318, not in Appendix D. Used to be Chapter 11, but my version of ACI-318 is old.

All I know is P/A and Mc/I

 

[WARose]

There are other failure modes with the anchors as well. (Pullout to name one.) But yes, you would also have to think about the overall failure mode of one/two-way shear in the slab. (Not to mention flexural forces and so on.)

 

[KootK]

Two tiered answer:

1) For loads that would push into the slab, a two way punching shear check is appropriate.

2) For loads that would pull away from the slab, ACI appendix D is all that you need and a punching shear check probably is not appropriate even if it would yield similar/conservative capacities in some cases.

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.

 

[Deener]

Thanks for all the useful responses. I have considered all the failure modes in Annex D and will also check for punching shear as the compression loads are significantly larger than the tensile loads. From my perspective, Annex D is covering the local failures of the anchors, where punching shear would be considered more of a global slab failure. Since we provide interface loads produced by our machine to the structural engineer for the building, I would think they will be checking punching shear. Still, I think it's a good check for me to do.
Thanks again