Punching Shear

Punching shear (two-way shear) is the brittle failure mode in which a column punches a roughly conical/pyramidal plug through a flat slab or footing, occurring on a critical perimeter taken at d/2 from the column face per IS 456 Cl. 31.6 (and Cl. 34.2.4 for footings). It is the governing — and most dangerous — check for flat slabs and isolated footings because failure is sudden, with little warning, and several real flat-slab collapses have been punching failures, often aggravated by unbalanced moment transfer at edge/corner columns.

IS 456 limits the nominal shear stress on the critical perimeter to a permissible value (a function of √fck, with a column-aspect factor); if exceeded, the section must be enlarged, concrete grade raised, a drop panel/column capital added, or shear reinforcement (stirrups, shear studs/rails) provided. The moment transferred between slab and column adds shear on part of the perimeter and must be included — a frequent omission. It is also the critical check for pile caps and the slab around heavy point loads.