Experimental and numerical study of flat plates retrofitted with inclined U-stirrups

As reinforced concrete flat plates lack beams, loads are transferred directly to columns. During an earthquake, the displacement-induced unbalanced moments generate shear forces at the flat plate-column connections. This combination can lead to brittle punching shear failure, especially in regions where seismic effects are not properly considered in design, or construction. Many existing flat plates in such areas require strengthening. Use of U-stirrups as shear reinforcement with strong epoxy is very convenient and effective in strengthening the punching shear capacities of flat plates due to ease of accessibility and placement. Based on critical shear crack theory and structural mechanics, shear reinforcement has been designed in the present study and a finite element numerical model is proposed for the purpose of analysis. In this experimental study, eight half-scale frame specimens with varying slab thicknesses and concrete strengths were tested under lateral cyclic loading and constant vertical load. The punching shear capacity was assessed through load-deformation measurements using dial gauges and a video extensometer. Finite element models have also been developed and verified with the experimental results. Good correlation is observed between the proposed and experimental results. Strengthened joints demonstrated improved performance under cyclic loading, while control specimens without shear reinforcement experienced brittle failure.