designing a shear key

huangyhg

designing a shear key
i'm just curious how one goes about designing a reinforced concrete shear key.  it wasn't anything i ever heard about in school and none of my textbooks even mention it.  what are some good resources?
more specifically, my problem basically deals with a rectangular concrete channel in the ground.  i need to know how to dimension the shear key to resist the forces due to soil pressures.  also, how would i take the shear key into account when determining the flexural strength of the connection?
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well, just think about this for a bit.  what the does the shear key resemble attached to the bottom of a retaining wall footing?  
as for load, you know it must be strong enough to mobilize the passive soil pressure without yielding.  and that it can't mobilize the passive soil pressure without pushing on the soil on one side and moving away from the soil on the other.  since the soil is without tension it won't hold the key back.  
so by now you probably are thinking you have a small cantilever from the bottom of the footing loaded on one side with the passive pressure.  
i think you can find the design of a concrete cantilever with a trapezoidal load on it in some text book.
regards,
qshake
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typically a shear key is put in place to allow for waterproofing elements (waterstops) to be placed prior to the next pour, in addition to strength requirements.   
this will typically just be a 2x4.
size the vertical reinforcing for the overturning moment, check the reinforcement for shear friction steel, and make sure you have minimum steel in both directions. if the channel is not huge- this would cover all bases. then the channel would be redundant, and in place to make sure you don't get a bunch of seepage that wasn't planned for at a construction joint (with waterstops in place).
i assume that you already know what the lateral force is, either from seismic or some or active load.
you geotechnical engineer should have provided you with a value for passive pressure and where it starts below grade.  
i usually start with a diagram for the passive pressure as some sort of triangular shape, depending somewhat on the soil parameters.   you can pick any width and start deepening the depth and calculating the resisting passive pressure versus the lateral force until you have a key deep and wide enough to meet your required factor of safety.
from there, you now know the forces against the key, which is a cantilever below the bottom of the footing and determine the bending moment in the key.  from there you can provide the necessary reinforcing.
this can be set up as a quick calc in a spreadsheet, substitute numbers into the input section and then having a output section that provides you with the demand-capacity ratios to guide you with how much you adjust the trials.
it sounds like we are talking about two different things.  an actual concrete extension into the soil under a retaining wall used to provide extra passive pressure for sliding resistance (qshake an oldrunner), and a formed groove in the top of the footing under the wall stem for waterstop placement (jen4950).  i believe he was initially refering to the former and therefore good advice by qshake.
jen4950-a slightly related question based on your point.  when you design a cantilevered retaining wall and check the shear friction across the plane, have you ever thought about the fact that there is only 2" or so of concrete cover adjacent to the reinforcing in the direction that the reinforcing bars will want to break out of the face of the wall?  it seems like aci's method for shear friction just assumes that the reinforcing can develope full yield despite having very little resistance against breakout on the side.  just thinking...
willisv
glad you brought that up.  i agree that concrete break out is a concern.  for that reason, i provide reinforcing steel through the joint on the opposite face of the wall.  i think of this steel as resisting the shear outright, so, in one sense, it takes the place of a shear key.  however, on the calc sheets i calculate this as "shear friction" steel.  the area of steel required to resist the shear outright is nearly identical to the area needed to develop shear friction.