shear key at base of walls

[vincentpa]

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?

 

[civilperson]

Shear keys are a waste of money. Cracks can form anywhere in concrete and if you imagine a crack that joins the re-entrant corners,(very likely), then you see only the reinforcement and friction is available to withstand horizontal shear.

 

[miecz]

See ACI 318, 22.5.4.

 

[structuralaggie]

Shear friction is the only resisting force that should be accounted for, but a shear key does make me feel better when I assume a roughened surface for this calculation. I don't trust contractors to roughen the surface sufficiently, and I think a shear key kind of accomplishes the same thing.

 

[AggieYank]

Shear keys are very easy to put in, which may be why they are so popular.

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).

 

[vincentpa]

I don't trust the shear key either. In VA, we used a roughened surface and "D" bars as called for by CRSI retaining wall. I never trusted a contractor to properly roughen the surface especially now that we have many illegals and non-english speakers doing to the work. I am not knocking their effort. they bust their asses working. I've just been out on the site and see that they just don't understand our construction methods very well at times. This goes for the Americans as well. Enough said about that. It is another thread entirely.

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.

 

[ironmon]

I agree with civilperson. It is now common practice to use and count on the vertical reinforcement for shear strength. Shear keys in walls are generally not used since the small benefit achieved does not merit the additional forming costs.
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

 

[structuralaggie]

I vote for good measure, yes, and use shear friction with roughened surface for total load.

 

[civilperson]

ACI 318, 22.5.4 applies to plain concrete members,(no reinforcement). The commentary says shear stress will rarely control, rather the tensile strength is controlling in plain concrete design.

 

[miecz]

vincentpa-

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.

 

[JedClampett]

My two cents is that if you're dealing with the wall slab joint in a tank, you're better off with a well prepared roughened joint and properly installed waterstop. Even if the joint is not well prepared, the shear friction value is usually of a magnitude that can carry the shear load. And above that, the key isn't doing anything anyhow.
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.

 

[vincentpa]

jmiec, sorry I didn't mention I was using reinforced concrete in my original post.

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!

 

[civilperson]

The liner is ESSENTIAL, pH of 2.5 will corrode and destroy the concrete and then the reinforcement. What provisions you use or do not use in the concrete will not matter if there is a leak.

 

[miecz]

vincentpa-

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.

 

[ron9876]

I always felt that shear keys, if they do anything, weaken the wall. Say if 12" wall with 4" key leaves 3-4" ears. Shear friction for me.

 

[DTGT2002]

Have any of you ever found a true reference for the determination of what constitutes 1/4" amplitude? Roughening to 1/4" sound reasonable on paper, but I have found no real guielines as to acceptablility or goals for the contractor ro attain. ICRI has some molded examples, but their examples are based on requirements for adhesion of coating, not necessarily shear friction.

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

 

[civilperson]

A light pounding with a bush hammer and then sweeping with a broom will remove laitance and leave a roughened surface of approximately 1/4" magnitude between the high spots and the depressions. Early is better than later, 12-15 hour old concrete is easiest to produce the roughnest desired.

 

[JedClampett]

DTGT2002, we develop the bars in the code prescribed methods. In the slab we use a standard hook. In the wall, we lap the dowels with the main vertical steel. we're actually more worried about bending moments, but we're also good for shear friction.
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.

 

[vincentpa]

I read when the concrete is still fresh, hitting it with a stiff broom will produce 1/4" amplitude. I just can't remember which article I read it in or if it was the ACI concrete repair seminar.