Predicting the punching failure of lightly reinforced concrete flat slabs without shear reinforcement

This paper presents an investigation into the punching failure of reinforced concrete flat slabs using nonlinear finite element (FE) analysis employing the smeared fixed crack approach and fracture-plastic constitutive models of concrete. It focuses primarily on lightly reinforced slabs where data is limited and discrepancies with design code predictions are identified. To this end, FE models were initially validated against three lightly reinforced slabs of varying depths reported in the literature. These FE models were then employed to predict the punching shear strength of six additional slabs and to conduct a parametric analysis of factors influencing punching shear. The FE predictions were also compared with those of punching shear design provisions and with theoretical calculations based on the yield-line theory. The results highlighted that the nonlinear FE models were capable of predicting the load-deflection response of the slabs and replicating the development of cracks in the concrete and the distribution of stresses in the reinforcement well. Although the majority of the slabs exhibited flexure-dominated behaviours, all were predicted to ultimately fail in punching, consistent with the mode of failure observed in the experiment. The results also indicated a shift in the failure mode, transitioning from flexural punching to pure punching shear, as the slab depth increased. The observed-to-FE predicted punching shear strength ratio for all slabs was 0.98 with a coefficient of variation (COV) of 9.5 %. The code predictions yielded ratios from 0.92 to 1.06, but this was skewed by the flexural capacity of the lightly reinforced slabs. For these slabs, the FE predictions remained the most accurate, with a mean ratio of 0.99 and 5.3 % COV, while the code predictions ranged from 0.61 to 0.88. Finally, the parametric analyses demonstrated that punching failure was more prevalent in thicker slabs, slabs with higher reinforcement ratios, and slabs using higher grades of reinforcement.