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Appendix D: Anchor shear breakout

Appendix D: Anchor shear breakout

Appendix D: Anchor shear breakout

(OP)
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?

RE: Appendix D: Anchor shear breakout

I believe edge distance begins to limit strength at 1.5 times the embedment.  That is the distance that the projected concrete failure area (D5.2.1), and the edge distance modification factor (D5.2.5) begins to be affected.

RE: Appendix D: Anchor shear breakout

(OP)
jmiec, D.5.2.1 and D.5.2.5 relate to tension breakout. My question relates specifically to the distance from the anchor to the free edge in the direction of shear.  

RE: Appendix D: Anchor shear breakout

The commentary says breakout will usually not govern, not that it won't govern.  You still have to check it.  If 50k is the answer after checking D6.1, D6.2 and D6.3 then it is what it is.  What size/type anchor and embedment are you using?

RE: Appendix D: Anchor shear breakout

Hokie-

Oops, you're right.  I don't see a limiting distance in ACI or the PCA Notes on ACI318.  Judging from the examples in PCA Notes, I believe that the research was done with small edge distances, less than 12 inches, and wouldn't apply to your case.  Also, I should think you would engage the mat reinforcing, which would preclude Appendix D altogether.

RE: Appendix D: Anchor shear breakout

(OP)
I appreciate your responses.
Here's an example that illustrates the problem I'm running into:
Code:  ACI 318-05
Single 2"-dia anchor embedded 20", located at center of 8'x8'x2'-thick footing.
F'c = 3000 psi
Edge distance = 48" on all sides
Effective Ca1 = max(48"/1.5, 24"/1.5) = 32" (influenced by 3 edges, see D.6.2.4, changed from 318-02)
Running the numbers, I get:
Av = 2304 in^2
Avo = 4608 in^2
l/do = 8
Vb = 148.8 kips
TAOec,ed,c = 1.0
Vcbg = 74.4 kips
phi*Vn = 52.1 kips (phi = 0.7)

Here's the thing that gets me:  If I increase the footing length in the direction of shear, I get the exact same answer, because my effective Ca1 is limited by the side edge distances.  If my anchor is say 20 ft from the free edge in the direction of shear, I'm pretty sure it's not going to fail in breakout, even though the numbers say otherwise.

 

RE: Appendix D: Anchor shear breakout

PCA Notes publishes  Table 34-6A with Shear Breakout Strengths for c1 up to 3 times the embedment and embedments up to 25 inches.  So I guess the formula applies up to 75 inches.  Looks like your 50,000 lb is right in the ballpark.

RE: Appendix D: Anchor shear breakout

HokieStructural - Yup I have run into this before. I agree with your numbers as well as your judgement that it is completely rediculous.

Generally for footings etc. jmiec his the nail on the head in that the breakout prism would cross the main footing reinforcing and therefore the shear breakout mechanism wouldn't really be valid - the strength of the main footing reinforcing in tension (provided it is properly developed on each side of the breakout prism line) would control.  

RE: Appendix D: Anchor shear breakout

The ACI anchor formulae are for plain concrete, reinforcement has a different calculation.

RE: Appendix D: Anchor shear breakout

Since no edge distance is less than 1.5c1, I don't think it is intended that you limit the embedment to c1/1.5.  Check the commentary RD6.2.4.  I think that will help a bit.

RE: Appendix D: Anchor shear breakout

you probably have some reinforcing between the anchors and the edge.  I believe this reinforcing would increase the breakout strength....shear friction

RE: Appendix D: Anchor shear breakout

You can calculate the edge distance required for the shear capacity of the bolt. That's your limiting edge distance. Anything larger and breakout will not govern.

RE: Appendix D: Anchor shear breakout

civilperson-
How do you account for the reinforcement like you mention in your post?

RE: Appendix D: Anchor shear breakout

Use the 35 degree cone from the anchor to intersect either the edge, face or side.  The area of this cone is what is used to prevent failure in the various modes.  When the cone area intersects a reinforcement bar, the component of the area of the steel x shear strength at right angles to the cone surface is available to resist the failure also.

RE: Appendix D: Anchor shear breakout

civil, where did you find that in the code, or any code?

RE: Appendix D: Anchor shear breakout

wouldn't you use the additional capacity gained by reinforcing passing through the shear plain.  I would calculate the capacity using appendix D and than add additional capacity from any reinforcing passing thru that failure plane using shear friction.

RE: Appendix D: Anchor shear breakout

UcfSE - see ACI-05 RD.4.2.1

"For anchors exceeding the limitations of D.4.2.2, or for situations where geometric restrictions limit breakout capacity, or both, reinforcement oriented in the direction of load and proportioned to resist the total load within the breakout prism,and fully anchored on both sides of the breakout planes, may be provided instead of calculating breakout capacity.

RE: Appendix D: Anchor shear breakout

"...oriented in the direction of load..."  That does not sound like the situation civil is describing.  civilperson is decribing bars crossing through the cone, like coming into and out of the pyramid sides, perpindicular to the load not parrallel to it.  

RE: Appendix D: Anchor shear breakout

Reinforcement that runs parallel to the edge will be at right angle to the load direction and hence will not count for design. In practice it will have some effect but it will be small.

RE: Appendix D: Anchor shear breakout

if you shear off a #5 bar that passes through the shear plane twice...2*.9*60*.31=33.48 kips...maybe small if your talking about 1000 kips..

RE: Appendix D: Anchor shear breakout

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?

 

RE: Appendix D: Anchor shear breakout

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.  

RE: Appendix D: Anchor shear breakout

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.

RE: Appendix D: Anchor shear breakout

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.

RE: Appendix D: Anchor shear breakout

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.

RE: Appendix D: Anchor shear breakout

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.

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