Shear in Mat Foundations

I have never used shear reinforcement in a mat foundation. A contractor once told me, "You can pour a lot of concrete into the ground for the same price as complicated reinforcement."

I think it IS correct to check one way shear across the entire width of the mat. I don't believe a mat can fail in one way shear along a length which is less than the entire width of the mat. But you must also check two way shear.

DaveAtkins

I've never used shear reinforcing in a mat either (I have in a pile cap before though). TO check the mat for shear, I've normally just done some hand calcs. Most of the time, two-way controls [typically for a column coming down to the mat] and the failure plane for that is fairly straight forward. For one-way, if the mat is supporting a wall or something I'll run some numbers on a per linear foot basis. As a back up to this, since most mats are modeled on some sort of elastic base, I've typically compared the shear stresses to the allowable(s).

In the particular case that I'm looking at thickening the mat would introduce underpinning of adjacent buildings so I need to compare that cost/risk to shear reinforcement. I am pretty sure reinforcement is preferable in this case.

I don't know that you can say that it can only fail along the entire width, there has to be some calculation or justification for that. I guess you'd have to show that you are able to transfer the shear in the perpendicular direction along the mat over that width. For reference this mat is about 70ft long and currently 4.5' thick with shear reinforcing. I did find a nehrp article that spoke about effective widths and shear reinforcing in mat but they essentially left it as up to judgement. Was just wondering if anyone had any other articles/references.

This is an excellent reference for slab / footing shear reinforcement: Link. It's British but still very applicable to North American practice.

One thing to consider is whether or not your raft footing will be poured in multiple layers. Thick ones often are. If that's the case you may want to have ties installed to provide horizontal shear transfer capacity between pours at the cold joints. If you do specify ties, then there will be little harm in installing some extra ties to serve as shear reinforcement. I've used shear ties that were 90 hooks at the bottom and 135 hooks at the top. As long as your longitudinal bar spacing isn't too tight, it seems to work fine. They drop the bars into the cage from the top, thread the 90 hook under the bottom steel, and then rotate the tie to suit the 135 hook at the top.

I totally get your concern regarding the effective width for one way shear. I have the exact same thought regarding one way shear in regular, two way elevated slabs. If you look at FEM results at 1/3 span, you'll find that the one way shear is concentrated in the middle of your design strip. Yet you design for one way shear base on the whole width of the strip. I wonder if one way shear is really a more ductile failure mode than we often assume and that lateral redistribution may in fact be possible.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

Good information on one way shear in this publication

Figure 10.1 of Slick's reference shows a solid tie detail as well.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.