Punching shear strength of RC flat slabs at interior connections to columns

Abstract

A method to evaluate the punching shear strength of reinforced concrete (RC) flat slabs without shear reinforcement at the connection to interior columns is proposed. The method is based on the assumption that the punching shear strength is controlled by the inclination of a unique punching shear crack that produces a conical failure surface. The inclination angle of the crack is variable. It follows that the location of the critical section is not established in advance, but changes with crack angle. Its location depends on the reinforcement ratio, material strengths and effective depth. According to the proposed method, the punching shear strength is a function of the inclination angle of the governing crack that controls the amount of shear carried by the compression zone and the flexural reinforcement crossing the potential punching cone by accounting for its slenderness and concrete size effect. The method describes the behaviour observed in tests and numerical and analytical investigations. The novel premise that the punching shear strength of flat slabs at connections to interior columns is controlled by the inclination of the failure surface shows remarkable agreement with the results of 209 tests on isolated specimens reported in the literature. This paper also assesses the adequacy of strength predictions obtained using the proposed method and the methods adopted in the codified provisions.