In flat slabs, which shear governs design in the critical region around columns?

• A. Punching shear around columns.
• B. Flexural shear in beams.
• C. Torsional shear around column corners.
• D. Total shear across the entire slab.

Answer: (A)

Punching shear around the column is the governing design check in flat slabs. When the slab transfers load to a column, the load is carried through a relatively small contact area, creating a concentrated shear stress that tends to cause the slab to “punch through” around the column. The critical region for this type of failure is a perimeter around the column, located at a distance from the face of the column (often tied to the effective depth of the slab), where the shear plane develops. The design must ensure that the resistance of the surrounding concrete, sometimes aided by shear reinforcement or a thicker slab or a drop panel, is enough to carry the punching shear force along that perimeter. This localized punching mechanism is what governs the strength requirement around columns in flat slabs, rather than the general shear that might exist along beams, or a torsional or whole-slab shear notion. In short, the danger is the column punching through the slab, so punching shear around the column region dictates the design.