Punching Shear in Foundations - Bending

[Luceid]

Hey all - I've got a concrete structure that we've designed using pinned-base assumptions with a mat foundation system where the columns are delivering vertical point loads at the base.

The SAFE model has a subgrade modulus spring support - so the mat foundation generates flexural moments as it deforms under loading. Many of these columns are near the edges/corners of the mat - so they often get some tricky moments in two directions which are being designed for flexurally.

When it comes to the punching shear of these columns through the raft - are the moments in the raft considered as part of the punching calculation as they are in elevated slab conditions with "restrained/fixed" slab-column collections? Or would you just take the axial load from the column and essentially check V/A - no component due to raft flexure? I have a tough time understanding the flexural component of punching shear in general - with this case I am even less sure what is clear.

 

[HDStructural]

I would think that you do not need to consider the moment. Similar to a spread footing loaded by a pinned column, there would be moment in the footing. We do not consider moment when doing the punching shear check in this case. For a two way slab, we would only consider moment with punching shear when a moment is transferred from the two way slab into the column.

Since no moment is in the column itself, I think no moment would be considered for the punching shear check.

 

[Luceid]

That's a fair point HD - thanks

 

[canwesteng]

Jumping in too late, but it is the portion of unbalanced moment transferred by shear that you design for. No moment in the pier means there is no unbalanced moment.