Isolated Footings - Use of Shear Key to Resist Horizontal Forces 1

[canstruct12]

I have been tasked with designing a spread footing for a large canopy structure over an existing outdoor rink. The contractor/client does not want to use a wall footing between the isolated spread footings. I suggested using a shear key to my supervisor and he said it was a good idea. After trying to do some research I cannot find any where that supports the use of shear key to resist horizontal forces for a spread footing.

Can anyone provide any insight to this?

 

[BridgeSmith]

The geotechnical resistance provided by the shear key is the passive soil resistance that is developed against it. I use the provisions of the AASHTO LRFD Bridge Design spec. to calculate passive soil resistance, but there are likely sources that explain the process better than AASHTO.

 

[raligt]

see P398 moment resisting joints to eurocode 3, page 72

 

[KootK]

- Your best bet is probably to look into the world of cantilevered retaining wall design as this is most often encountered there.

- I'd run this concept by your contractor before sinking too much effort into it. My experience has been that contractors dislike shear keys.

- If you're in north america, a good retaining wall resource covering this is the CRSI manual.

 

[WinelandV]

You could always size the footings to resist the horizontal forces via friction between the footing and soil.

 

[msquared48]

Or just bury the footing deeper to pick up soil load above and increase the passive resistance too.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

Shear keys would be a bit unusual for spread footings. Friction usually is sufficient. If there is lots of uplift, then you usually want a grade beam to help control moments.

But, I see no reason why it can't be done. As KootK suggests, I'd look at retaining walls. After all, retaining wall foundations are really just a special case of spread footings. And, that's the one application where shear keys are used the most.

 

[jayrod12]

I'm personally a fan of just making the spread footing as deep as required to resist the lateral forces. The finicky excavation and the rebar detailing for a shear key seem to be a waste of money when concrete is only 250-350$/yd^3.

 

[KootK]

The finicky excavation and the rebar detailing

Yeah, especially since your shear key is likely to look something like this.

 

[WinelandV]

KootK,

At least the shear key is deep enough.

 

[jayrod12]

shear key is likely to look something like this.

If you're lucky it looks that good.

 

[BridgeSmith]

KootK, the shape of the shear key depends on the material being keyed into (and how good the QA is). For our part, we only pay for what we we detail, and our field inspectors watch the contractors fairly closely. I don't believe they'd get away with something that looks like your sketch on one of our projects. Of course, if the foundation material was that poor, we would likely not consider using a spread footing, anyway.