Concrete Breakout Strength for anchors in tension

You're looking at the capacity of the anchors as a group, correct? I believe the appropriate way to look at it would be to look at edge distance for the group. In other words, your maximum would be 8".

I don't have a copy of 11 anymore, but this is word for word to Chapter 17 in the 2014 edition. (17.4.2.3 to be precise.)

If you look at the commentary for that section, it provides a visualization to help understand the requirement.

ACI 318-14 R17.4.2.3 said:

The requirement of 17.4.2.3 may be visualized by moving the actual concrete breakout surface, which originates at the actual h[sub]ef[/sub], toward the surface of the concrete parallel to the applied tension load. The value of h[sub]ef[/sub] used in equations in 17.4.2.1 through 17.4.2.5 is determined when either: (a)the outer boundaries of the failure surface first intersect a free edge; or (b) the intersection of the breakout surface between anchors within the group first intersects the surface of the concrete.

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Ca,max for this case is 8".


I see that you using 22" since that is the distance from one anchor to the opposite edge of the pier. This would not be consistent, though, with the example that they give in Fig. RD.5.2.3, since one of the anchors would be 14" from an edge using your method, but they use 6". I think you're supposed to consider how the entire group breaks out instead of each individual anchor. The critical dimension between anchors is their spacing, not the distance to the edge beyond the adjacent anchor.

Using 8" also makes sense when you consider the purpose of the provision. If you have an embedment of 5.33", then your breakout area for the group, Anc, is the entire area of the pier. If your embedment is any deeper than this, Anc will not increase any more, but the breakout area for a single anchor that is not limited by distance, Anco, will continue to increase as hef increases. This will result in a lower tension breakout strength, Ncbg, (since Anc/Anco is the first factor) even though all you're doing is adding more concrete over the anchors which should not affect the capacity.

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I would just reinforce the breakout cone in this scenario, but if you're actually doing the math, isn't the breakout capacity for the group just

(breakout area of group / breakout area of one theoretical anchor not near an edge) * breakout capacity of theoretical anchor not near an edge

So if you know that the breakout area is the whole pier and you don't have any weird eccentricities, can't you just skip the geometrical back calculating of the edge distance and spacing terms and just use the area of the pier to figure it out?

Been a bit since I've had this specific scenario by hand, though, so don't quote me on it.

phamENG,

This foundation supports a "flagpole" column that is one in a series supporting cable trays. The controlling lateral force is from wind. Cable trays are 20+ feet above grade. Only two at a time of the four anchors are in tension, so maybe it should be analyzed for breakout due to tension as if there are only two - which would be located offset 7" from centerline of pier.

ProgrammingPE, Then would it make more sense to consider Ca,max to be 22", thereby ignoring the presence of the anchors not in tension?

Thanks, everybody, for your input.

For an applied moment, you can ignore the two anchors that are not in tension. You would still be using a ca,max of 8", though, since the group with only two anchors in tension still breaks out at three edges at that depth.

(See the wording used in the commentary RD.5.2.3: "ca,max is the largest of the influencing edge distances that are less than or equal to the actual 1.5hef.")