Punching Shear perimeter for corner/edge Columns on a Mat foundation

[Ashfaq S]

Hello Everyone,

I have a column located close to the corner of the mat foundation. The edge is close to 4d from the face of the column, where d is the distance to the tension reinforcement from the compression face. I am trying to check the punching shear for the foundation using the Canadian code.

For a two way slab supported by a column, the punching shear perimeter is limited by the following assumption:

​

CSA 23.3-14 states that if the free edge is less than 5d from the face of the column, then the side is considered ineffective for punching.

​

ACI 381-19 has a similar limit for free edge less than 4h from the face of the column/

Now, when it comes to checking the column punching shear for a mat foundation, do these edge limits still apply and the perimeter is only effective on two sides (Option 1)? or Can I assume the punching shear perimeter to be effective on all 4 sides of the columm as long as the edge is located (say) 2 or 3d from the face of the column (Option 2)?

I am aware that I can reduce the column force with the force due to the bearing pressure on an area enclosed by the punching shear perimeter, how do we deal with this for corner/edge conditions if the two way slab limits apply?.

Note: In Option 2, CSA limits the punching shear perimeter to extend "d" from the face of the column located near the free edge. ACI allow this to be extended all the way to the free edge.

 

[KootK]

In my opinion, the same 5d/4h business that would apply to an elevated slab is also applicable to a raft slab. While I don't know that it addresses the particular issue raise in this thread, this NEHRP is one of my current favorites when it comes to mat foundation design: Link It hits on some interesting aspects of one way shear etc that I've not seen addressed in other places.