What are the factors affecting the punching shear?
Punching shear is produced directly by the forces and moments acting on the column. For the simple case of a concentric footing with a downward load, the punching shear can easily be calculated as the volume of the bearing pressures on the critical area around the column. However, in the presence of overturning moments the calculation may become very cumbersome since the shear is no longer uniform along the critical section.
This is similar to the case of an elevated flat plate supported by a column, where the unbalanced moments will produce a shear at the slab-column junction. The factored shear stress on the critical section is the sum of the direct shear and the shear caused by moment transfer, as shown in the figure below.
In this expression γv is the fraction of the moment being transferred by shear and J is the polar moment of inertia of the critical section. A third term may be added for the moment in the other direction. Since both Vu and Mu are involved, it’s better to compare the punching shear to the shear strength in terms of stress, rather than force or moment.