huangyhg

sear key
the question is regarding functioning of shear key used in the footings. i have a square footing (8'x8') with a large horizontal force acting on it and it fails in sliding. to help resist in sliding, i decided to put a shear key 2'x1.5' at the bottom of the footing. now, to resist sliding, there is a frictional resistance force 'mu*summation of all vertical forces including the weight of soil and concrete', the passive pressure of soil acting at the back of the shear key and also the depth of the footing. is the shear key itself capable of resisting some of the horizontal forces say by use of shear stirrups?
i am looking forward to your comments.
thanks,
robert
are you asking about designing the reinforcement for the shear key, or how the shear key helps resist sliding forces?
the shear key increases your depth for developing passive presure resistance to sliding. you will have a linearly varying force on the face of the shear key itself, causing and internal bending moment and shear force. both of the must be accounted for when you design the thickness and reinforcement of the shear key. keep in mind that this is a footing, basically a hole in the ground. you shouldn't try to get too precise by adding too much steel that isn't normally found in a typical footing. you're better off increasing the thickness of the key to resist shear so that you won't need stirrups. you will likely need some reinforcing, if only minimum, to resist the bending moment in the key.
it looks like you got the point of providing a key, but your question isn't quite clear on what you mean, to me anyway. stirrups will help with internal forces induced by the external sliding force, not with the sliding force itself. hope this helps
shear keys dug in soil sound alot better than they truly are. the main reason is that when the key is dug out by hand lets say, that process disturbs the sides of the soil thus your passive pressure may not be as good as you typically would design it. while the key clearly helps reinforced as you are designing, it may take a lot sliding from the footing to occur to actually mobilize the passive pressure benefit a situation you do not want to happen. what is your column load and what type of soil friction factor are you using?? do you have a concrete slab over the footing?? i would use a key only under a special inspection requirement during the excavation of the key and the pouring of the spread footing, otherwise i would not count on it. hope this helps. good luck.

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