shear key at base of walls
shear key at base of walls
(OP)
Does anyone know a rational method for determining the capacity of shear keys at the base of walls to transfer shear from the wall to the base slab or footing?
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shear key at base of walls
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RE: shear key at base of walls
RE: shear key at base of walls
RE: shear key at base of walls
RE: shear key at base of walls
I typically use the shear strength of the width of key divided by 2 as my shear capacity strength. I divide by 2 because the shear key is never perfect. Maybe it's only 1" deep, maybe it cracked like civilperson said, maybe it's going to be half full of waterstop, etc.
Even using the full strength of the shear key, it often isn't enough to resist shear. For example, at the base of a retaining wall, one typical detail is to pour the footing with a continous shear key where the wall will be poured on top. This shear key isn't enough to resist the shear at the base of the retaining wall assuming the wall is decent size. I've changed our typical detail to add dowels @ 12" o.c. (usually #4 or #5, depending).
RE: shear key at base of walls
So you're saying, put the shear key in for good measure, use "D" bars as the CRSI retaining wall examples. I like it. I am designing large tanks (36'x36xx28') and a large clarifier (180' in diameter). I am concerned about the base details. I am really concerned about the temperature and shrinkage and how it will affect the base.
RE: shear key at base of walls
The method is called shear friction outlined in ACI-318 11.7
When you think of it Masonry wallv do not use shear keys, they rely on the dowel action of the vert reinf.
However the shear strength along the key = 0.85*2*sqrt(F'c)*b*d if b =12" then that's the ultimate shear strength per foot
RE: shear key at base of walls
RE: shear key at base of walls
RE: shear key at base of walls
Your original post didn't say you were dealing with reinforced concrete. In that case, I agree that a shear key is useless. Worse yet, it can contribute to consolidation problems and leakage, especially when combined with a waterstop.
RE: shear key at base of walls
We use no keys and starter walls to stand up the waterstop above the slab reinforcing. I've never seen a shear failure in this detail, but I don't want to get too cocky.
RE: shear key at base of walls
I need to use an upturned key just to keep my waterstop from interupting the rebar in the top layer of my base slab. This tank is holding acid mine drainage with a pH of 2.5 sulphuric acid. Nothing can get out. I am lining the tank but just in case... I think I will use a relatively short upturned key, say 2x4, just to keep the waterstop from interfering with my base slab bars but I am going to roughen the remaining surface to a full 1/4" amplitude and use dowel bars. The walls are 26" thick on this tank. I don't think there will be a problem with the shear friction. However, I do have some smaller tanks where I won't have such a large contribution from a thick wall.
Thanks for all of you posts!
RE: shear key at base of walls
RE: shear key at base of walls
Agree with JedClampett. You are better off without the shear key. Putting a waterstop in a turned up key looks OK on paper, but is a pain to construct and consolidate. To eliminate the interference between the waterstop and the top reinforcing, bend down your top reinforcing slightly near the inside face of your wall, and locate the waterstop close to the outside face of the wall.
RE: shear key at base of walls
RE: shear key at base of walls
Further, what mechanisms are typically in place to verify full development of either side of the roughened face? You can't could on shear friction at all without developed bars.
Anyway, any guidance would be great, or at least knowledge of what others find reasonable would be nice.
Thanks,
Daniel Toon
RE: shear key at base of walls
RE: shear key at base of walls
I don't really worry about the mechanics of the roughened surface that much. Unless it's troweled smooth, it's rough enough to me. The chemical reaction of the concrete hydrating passes through the joint over time and it acts pretty much monolithically.
RE: shear key at base of walls