Shear Breakout for Anchors according to ACI 318

[SixkHz]

I've got a question about determining the concrete breakout for shear loading according to Appendix D of ACI 318. From the attached sketch, I have a steel column (which is part of a braced frame) anchored to a concrete T-beam below (24" beam + 8" slab). The shear load is pretty big and I'm getting the section to fail under concrete breakout, but I'm not sure about my breakout area calculation.

In reference to Section A-A from my sketch, I've used the edge distance from the anchors to the edge of the concrete beam, but this neglects the continuity of the 8" slab above, which I would think would add significant shear resistance.

The beam is heavily reinforced, so I think I'm just going to rely on the reinforcement to restrain breakout and check it as per Chapter 12, but I'm curious as to how you'd arrive at a concrete breakout resistance for this type of section.

Any input is appreciated.

 

[sandman21]

It appears that you are going about it the correct way.

App. D properly accounts for shear failure when you are close to an edge when you get away from an edge the equations get a little screwie there is technically no exception for shear breakout like there is for concrete breakout. I would add additional rebar developed into the slab to take the shear load.

 

[ATSE]

Don't ignore the 8" slab.

 

[gobsmacked]

If you can get some positive transfer from the base plate into the slab using a downward fin on the base plate, attached to a rebar connector, and do this both ways into the slab, then the shear problem you describe goes away, doesn't it?

 

[slickdeals]

I would just provide reinforcing developed past your last line of anchors and developed into the slab to carry the shear force.

ACI 318-08 has some good sketches in Appendix D.

 

[a2mfk]

You may also want to consider a shear lug for the shear (HSS or WF welded to the bottom of the base plate) and then use anchor rods for the tension. You can then design it for a bearing shear failure (similar to your failure sketches), but you can reinforce accordingly, similar to a corbel... This gets you out of Appendix D for the shear design. No matter what I like a couple of stirrups placed through the shear triangle and the legs extend back into the structure.