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?

 

[miecz]

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.

 

[HokieStructural]

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.

 

[UcfSE]

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?

 

[miecz]

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.

 

[HokieStructural]

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.

 

[miecz]

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.

 

[WillisV]

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.

 

[civilperson]

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

 

[UcfSE]

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.

 

[par060]

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

 

[sdz]

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.

 

[Lion06]

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

 

[civilperson]

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.

 

[UcfSE]

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

 

[par060]

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.

 

[WillisV]

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.

 

[UcfSE]

"...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.

 

[sdz]

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.

 

[par060]

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..