Seismic design of concrete slabs for punching shear: a critical review of Canadian standard CSA A23.3-04

Unbalanced moments, transferred between concrete flat plates and columns due to earthquakes, produce shear stresses that increase the vulnerability to brittle punching failure. Appropriate design and detailing of the slab-column joints are essential to prevent such failure. Because of the high flexibility of flat plate structures, lateral-force-resisting systems, e.g., bracings, frames or shear walls, are necessary to limit the interstorey drift ratio to specified values. The slab-column joints must have the strength and the ductility to undergo, without punching failure, the lateral displacement of the structure. In the 2004 version of the Canadian standard, CSA A23.3-04, clause 21.12.3 is added, specifying the punching shear design of slab-column connections subjected to earthquakes. The provisions of the clause are critically reviewed using a design example. It is shown that the new clause permits absence of shear reinforcement in cases that lack strength and when it requires shear reinforcement, it specifies excessive amount. Changes to the standard are proposed.