What is punching shear?
The failure mechanism of structural elements like slabs and foundation etc. by shear, is called punching shear. Consider a portion of slab subjected to an increasing concentrated load. Eventually the slab will fail. One possible method of failure is that the load punches through the slab.
Some examples of the occurrence of concentrated loads on a slab are a column, particularly on a pad foundation, and wheel loads. This same type of failure could also happen in another way. Turning the structure upside down we get a flat slab supported by a column, where there is a high concentration of shear force around the column head.
When the total shear force exceeds the shear resistance of the slab, the slab will be pushed down around the column, or this can be viewed as the column being punched through the slab.
Punching shear is most common, and is a major design consideration, in flat slab construction. In pad foundations, where weight and depth are not so critical, its effects are satisfied by providing sufficient depth.