Appendix D: Anchor shear breakout

[HokieStructural]

The commentary for Section D.6.2.1 states that shear breakout will not control for anchors that are "far from the edge." I'm looking for guidance on how far an anchor should be from the edge in order to neglect shear breakout. For example, I'm calculating a shear breakout capacity of only 50 kips for an anchor in the middle of an 8'x8'x2'-thick footing. Judgement tells me this is rediculously conservative, but the numbers say otherwise. Has anyone else dealt with this?

 

[WillisV]

UcfSE - I agree that specific provision is for bars oriented in the direction of the load - I didn't interperate civil's post correctly on the bar orientation.

However, I don't see anything in the code that would disallow using the bars to reinforce the breakout cone in the manner referred to (running 90deg thru the breakout prism). After all wouldn't that just be shear friction using the "monolithic" friction coefficient?

 

[UcfSE]

I think it would be shear friction in locations far from an edge and where you can anchor the bar each side of the cone. Some engineers I have talked to or read posts from (not necessarily this thread) would suggest using shear friction on a bar with 1.5 inches of cover with load toward the free edge. That I don't agree with.

 

[sdz]

Major problem with bars running paralel to edge and hence at right angle to direction of load is that you would get your concrete breakout while mobilising very little of the steel strength. Even if you could prove that the steel oriented that way could take the load you would have a serviceability failure long before that happens.

Anyway the bars won't be in pure shear. They will deform into an S bend and develop tension and bending; if they don't pull out themselves.

 

[vincentpa]

I asked an ACI guy that was on this committee at the ACI-05 Seminar about Appendix D particularities. I stumped this fellow on several questions about Appendix D that just don't feel right. He did not have any answers. One of the questions was this the one you ask. Another was how do you use supplemental reinforcement to develop both sides of the failure plane in (I had sketches). Do you require lap requirements on each side of the failure plane? They show a bar hooked around the bolts. Does this mean the bar is developed on each side of the plane? I asked him what to do if the pier is round, again no answer.

The committee guy asked me to send him sketches with my questions. I haven't gotten around to it. That was a year and a half ago. Shame on me. Apparently there is one guy, yes 1 man, that is the expert on ACI Appendix D and could answer all of these questions. I didn't get his name from the committee guy. I have 75% of my sketches and questions done. If I ever get around to it, I'll post the answer I get.

Once again, kudos to the ACI committee members for not knowing what the #!@% is going on in their committe.

 

[vincentpa]

See AISC Design Guides 1 & 7 and PCA Notes on 318 for more use and calculation of capacities. AISC does a better job of explaining supplemental reinforcement and provides better details. You will still have some questions after reading those but for most cases, things will clear up. The only case that AISC does not clarify well is supplemental shear reinforcement. They recommend using shear lugs.

 

[Taro]

vincentpa, Ronald Cook of University of Florida is probably the person you want to ask. He authored the 1999 PCA "Strength Design of Anchorage to Concrete" that was the precursor to the ACI Appendix D. He was very quick to respond to a question I had a few years ago when the PCA guide came out. Basile Rabbat of PCA was also helpful.