Shear in Mat Foundations
Shear in Mat Foundations
(OP)
Does anyone have any good references for designing for shear in mat foundations. In particular I am talking about large mats that support multiple shear walls and columns and are not easily divided up/visualized in any particular grid.
- Usually we provide a thickness such that shear reinforcement is not required. However this often results in huge mats and I wonder if this actually makes sense.
- What effective width to use for one way shear. With a huge mat it does not seem reasonable to take the entire width but is there any guidance on where to stop?
- When using shear reinforcement would it make more sense to use bent up bars rather than hooks. You get increased capacity and it seems like they may be easier to place in a very congested mat.
- Usually we provide a thickness such that shear reinforcement is not required. However this often results in huge mats and I wonder if this actually makes sense.
- What effective width to use for one way shear. With a huge mat it does not seem reasonable to take the entire width but is there any guidance on where to stop?
- When using shear reinforcement would it make more sense to use bent up bars rather than hooks. You get increased capacity and it seems like they may be easier to place in a very congested mat.






RE: Shear in Mat Foundations
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
RE: Shear in Mat Foundations
RE: Shear in Mat Foundations
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.
RE: Shear in Mat Foundations
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.
RE: Shear in Mat Foundations
RE: Shear in Mat Foundations
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.