Another question on ACI 318 App D

[Lion06]

Would you ever expect concrete breakout to control in shera for an embed plate in teh middle of a wall, even a very thin wall?
I am picturing a wall supported by a footing. I don't see concrete breakout (for shear) even being a failure mechanism for this case, becuase teh wall is supported in the direction of the load. If the wall were hung from above (I know that's not realistic), I could see it failing like that. I just don't see an embed plate failing in concrete shear breakout in a wall.
Does anyone agree or disagree?
The problem I am having is that when you run through the numbers for it, Avco gets very big as you start getting large ca1 distances (as is the case for a wall with an embed plate 10' from the base of the wall). The problem is this, as ca1 gets large, Vb gets larger and Avco gets larger, and Avc also gets larger, but it adds very, very little to the capacity. Also, if you have a wall where the embed plate is 10' from the base, and in the middle of a 12'wide wall, suddenly your breakout is affect by three sides (technically) because ha (wall thickness), ca2, and (what I call) ca3 (which is the edge distance on the opposite side of ca2) are all less than 1.5*ca1 and you need to do all these other things. I just don't feel like this was meant to be the case.

Any thoughts?

 

[frv]

StructuralEIT-

I ran into similar issues last year. I griped to anyone who would listen that it doesn't make sense.

The problem is that essentially your shear capacity is proportional to Vb/Avco

Vb is a function of Ca1^1.5 and Avco is a function of Ca1^2.. see the problem?! The further away you are from the edge, the LOWER this ratio is.

However, ACI 318-08 has addressed this issue.

I suggest you purchase 08 if only for the improvements to Appendix D. Included in these improvements is a method to increase capacity by taking advantage of reinforcement existing in your concrete member.

 

[Lion06]

Good to know that I'm not alone. Do you even see shear breakout being a possible failure mechanism for a free standing wall? I'm having a hard time picturing how it could fail in that manner when the load is in the direction of the support and the support is continuous (as in the case of a footing).

 

[WillisV]

Also see thread167-185091 where this limit state in similiar situations was discussed before.

 

[Lion06]

Alright, well here is the specific problem I am coming up against. If I have a 9 bolt pattern with 6.5" spacing perp to load and 10.5" spacing parallel to load. I am affected by 3 edges, with a ca1 of 24" which ends up turning into 34.33" taking into account the other considerations.
I get a pryout capacity of 124K, a steel capacity of 68K, and a shear breakout capacity of 8.8K (no, that's not a typo). That can't possibly be right, can it?
The math is definitely right, it just doesn't make sense.

 

[Lion06]

The thickness of whatever it is connecting to really seems to be the controlling factor. When the commentary says that anchors not near an edge will usually not govern seems to be the opposite of what I am finding. As I make the anchors farther from the free edge I am getting smaller capacities because of what frv mentions above.
I've never said this before, but this is completely assinine!

 

[ash060]

If you check the PCI Handbook they have a method for studs "in the field" of the concrete (away from all edges). It is in section 6.5.7 of the PCI handbook 6th edition. The equation is not a function of edge distance.

It does have a limitation for certain h/d ratios.

 

[Lion06]

If you follow ACI App D's guidelines very explicitly, you'll never get an embed plate in a 12" wall to be good for anything more than roughly 40K. I have an example in front of me for a 12" wall with (16) nelson studs spaced at 4" in both directions with 9" embed (I know that's high for nelson, I'm just trying to make a point). I'm getting a steel capacity of 185K, pryout capacity of 147K, and a shear breakout capacity of 40K. You can't make a plate much bigger or fit many more studs on than that.

 

[COEngineeer]

StructuralEIT, I had the same problem. I used RAM connection and the results were not making any sense. Went through it by hand and the program didnt make a mistake. It just did whatever appendix D is telling us to do.

Never, but never question engineer's judgement

 

[COEngineeer]

I am sorry.. I meant I used DS anchor for the program. I really think they only do a test on thin wall closer to the edge. So their equation is not very accurate when you put it really far from the edge. I tried to find a fine line when to ignore this concrete shear breakout but having a hard time.

Never, but never question engineer's judgement