Concrete Breakout in Shear

[spaseur]

I have a unusually condition where I am anchoring into an existing side of an concrete beam. The the center of the anchor plate aligns with the center of column above and below the beam. I would have said I am anchoring into the side of the column except the column is set back 2" from the face of the beam. The beam is 16.5" deep. I have 4 anchors two anchors per row. The top row and bottom row of anchors is located 4.5" from the edge of the beam to miss the longitudinal steel. The question is this, how would you determine the concrete break out strength in shear for this connection. See link. Do you ignore the column below and look at it as if it is just a beam - very conservative. Ignore the 2" offset - maybe not conservative enough. Cracked or uncracked?

 

[KootK]

1) Handsome detailing. That's going into the vault.

2) I hear you in that, surely, most possible failure modes would result in the failure frustum intersecting the column in a beneficial way, even if that benefit is difficult to quantify.

3) My first choice would be to make the beam work without consideration of the column but, since you're here asking this question, no doubt that strategy doesn't work.

4) With regard to shear breakout, I recommend the model shown below wherein the following assumptions have been made:

a) No true shear breakout mode is realistically possible.

b) In modifying the location of the center of the shear reaction, one ought to account for that in the pryout failure mode.

c) The blue frustum shown below is a reasonable way to account for the impact to the pry out failure mode if one adds in the shear load eccentricity implied by the model.

5) As much as it is within your power to do so, I think that it would behoove the connection to force as much of the load through the upper set of anchors as possible so as to give the load more time and space to strut its way over to the column.

 

[KootK]

Cracked or uncracked?

I vote cracked. Some things to consider:

1) It's the location of peak hogging moment in the beam. Point cracked.

2) If column ties run through the joint, they may help restrain cracking. Point, sort of, uncracked.

3) Most of your shear transfer will occur in the 2" of concrete outside the column cage. Point cracked.

4) If you decide to rely primarily on your low anchors for shear transfer, those would be closer to the beam compression block. Point uncracked.

 

[spaseur]

Thanks for the input. This question has come up due to existing conditions not clearly shown on existing drawings. That is the reason for the detailing in the sketch. I think I can make 6 anchors work - 3 in top row. But two of the anchors will fall outside the column bars. We like to keep them inside the column bars where possible.

As far as the cracking goes something I might have added. This is not a moment frame. It is at the end condition of a pan joist system. The joist is only 16.5 inches think. I do not anticipate much of a negative moment at this location.