I have always used the formulas in Appendix D to calculate the strength of anchors in breakout, pullout, blowout, etc. Just yesterday I was designing anchors for a slab, and the problem that I'm getting is that even if I put the anchors all the way down to the bottom of the slab, I'm still getting a breakout failure. This didn't make sense to me because I thought that if the load is that high, then shouldn't the slab also have a punching shear failure? After all, isn't breakout just a form of punching shear? So I checked the punching shear of the slab using the standard 2 way shear formula in ACI 318, and sure enough it passes. Can someone please explain to me why the formula for breakout results in way less capacity than the formula for punching shear. In my mind, an anchor with a washer on the end is no different than a bearing plate sitting on a slab. So why is it that something bearing on the outside of the slab is given way more capacity? Thanks for any assistance.
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