Performance and Design of Punching-Shear Reinforcing Systems

Punching shear reinforcement is increasingly used in flat slabs as an effective solution to increase their strength and deformation capacity. Several punching shear reinforcing systems have been developed in the past, such as studs, stirrups or bent-up bars. The efficiency of such systems is strongly influenced by their development conditions (anchorage, bond) and detailing rules. Codes of practice, however, do not typically acknowledge such differences, proposing the same set of design formulas for all systems. This approach is detrimental for some systems (with better detailing rules and anchorage characteristics) and does not provide enough guidance for design of others (not respecting codes’ detailing rules). In this paper, the fundamentals of the critical shear crack theory are explained with respect to the design of punching shear reinforcing systems. It is shown that this theory provides a consistent basis for design of shear reinforcing systems accounting for their particularities and modes of failure. The results of 6 tests on full scale slabs (3.0× 3.0× 0.25 m) with same flexural and shear reinforcing ratio but with different punching shear reinforcing systems are presented and discussed. The experimental results confirm that the strength and deformation capacity are strongly influenced by the characteristics of the shear reinforcing system. The results for the various systems are finally investigated within the frame of the critical shear crack theory, leading to a series of recommendations for design