Edge Distance for Concrete anchors 1

[NS4U]

In Appendix D for shear breakout, I cannot find where ACI defines what a free edge is. ca1 is supposed to be the distance from the anchor to the nearest edge.

I have an anchor that is 100" away from the edge (ca1) and ca2 is 9"... because ca2 is so small it is killing my capacity when using appendix D because Avc/Avco is around 0.4.

However I have read in ACI and PCA notes that if the anchor is not located near a free edge then you don't need to check shear breakout... but they don't define what a free edge is... is it heff? 1.5*heff?

I find it impossible to believe that my 3 anchors will shear off 100" of concrete before they themselves yield in shear... but strictly following ACI App. D that's what I'm getting, the steel shear strength is 200k, the breakout strength is 100k.

I think, if anything, I'd need to check shear breakout parallel to an edge for this configuation

 

[Lion06]

I actually looked into this pretty extensively for a spreadsheet that I wrote. I wasn't able to find very much on the subject, but I will share what I did find.
Like you, I found in PCA Notes that you don't have to check shear breakout if it isn't toward a free edge (however, ACI doesn't explicitly state that. In fact, ACI seems to say it needs to be checked no matter what).

What I have come to conclude the meaning of a free edge is, is just that - a free edge. Not being toward a free edge would be something along the lines of an embed plate in a wall supported by a footing (or another wall that is supported by a footing). There is no way for that to fail in shear breakout because there is no free edge. If you have a very far edge distance, but it is still a free edge, then you can potentially have a shear breakout failure.

I did find a paper by Edwin Burdette and Nikola Zisi that addresses this issue for thin members with large edge distances and they propose an alternate to the App D provisions because they do come across so conservative.

Additionally, it should be noted that if you have a very large ca1 distance, but only 9" for a side edge distance, you should be checking it for ca1=9" and doubling that value to get the shear breakout capacity per D6.2.1(c) (in ACI 318-05).

 

[NS4U]

Additionally, it should be noted that if you have a very large ca1 distance, but only 9" for a side edge distance, you should be checking it for ca1=9" and doubling that value to get the shear breakout capacity per D6.2.1(c) (in ACI 318-05).

I agree, this what I'm calling as checking breakout parallel to an edge.

It seems to me that as ca1 tends towards infinity, your capacity actually decreases, I would think ACI would have a limit for ca1 somewhere

 

[Lion06]

Sorry, I didn't notice your last line. The capacity only decreases as ca1 increases to very large values if your side edge distances stay the same and you have the issue of three sides affecting the breakout. As long as your side edge distances are greater than 1.5ca1 then you will get more capacity with increased ca1 distances. I do agree that the capacities seem very low - especially for members that are thin compared to the ca1 distance (e.g. a slab, or even a footing).
For example - An embed plate with 4 - 1" diameter headed studs embedded 8" into a 12" thick slab (or footing) having a ca1 distance of 2500" (which is 208') only has a shear breakout capacity of 215.2K. This does assume 4000 psi cracked concrete with no supplemental reinforcing and a 6" spacing of anchors in both directions, which is certainly a realistic scenario.
If you say the concrete is not cracked, and add supplemental reinforcing to this scenario, you get 322.7k. That still seems low for 208' of edge distance. You need 269" (22.4') of edge distance just to develop the strength of the anchors, which is 106k.

 

[StruKturdg]

What if you have a drilled pier or pedastal, that has ties spaced close at the top. Why can't we account for the shear ties in helping out concrete blow out?

 

[Lion06]

You can absolutely size rebar to take ALL of the load. There is nothing wrong with that.

 

[NS4U]

Check out ACI 318-08... they address using anchor reinforcment... the location and the anchorage of the reinforcment is a very big deal however

 

[JAE]

Wouldn't your condition fall under D.6.2.1(c)?

 

[unclesyd]

Here is a paper that may help. Start at Adobe page 105.

http://www.risatech.com/products/dnloads/risatower/Technical_Manual_1.pdf