Improvement of punching shear capacity of existing flat plates by using shear reinforcements

Abstract

As reinforced concrete flat plates do not contain beams, all the loads on slabs are transferred directly to the columns. During an earthquake, the displacement-induced unbalanced moments causes shear forces at the flat plate-column connections. Due to transfer of shear forces combined with induced unbalanced moments between slab and column, brittle punching failure can occur. Because of increase in applied loads and/or lack of consideration of seismic effects during design or construction, a significant number of existing flat plates are currently required to be strengthened against punching shear to avoid brittle punching failure. From literature review, it is observed that use of U-stirrups as shear reinforcements with strong epoxy is very convenient and effective in strengthening the punching shear capacities of flat plates. Based on critical shear crack theory and structural mechanics, shear reinforcements have been designed in the present study. Due to ease of accessibility and placement, U-stirrups are used for retrofitting flat plate column connections. In this experimental study, eight numbers half scale frame specimens were tested under lateral cyclic loading to observe the punching capacity of flat plates. The test was loading control so that the horizontal hydraulic jacks were used for imposing the cyclic loading. The specimens were subjected to incremental cyclic loading provided by hydraulic jacks under constant axial or gravity load and their load-deformation behavior was measured by dial gauges and video extensometer. The behaviors of the strengthened flat plate column connection are compared to the control models to observe improvement of punching shear capacity. The joints without shear reinforcement underwent brittle failure under cyclic loading, but their ductility increased with increased concrete strength. The joints with smaller flat plate thickness strengthened with shear reinforcements showed equivalent load bearing capacity as compared to that of greater plate thicknesses with enhanced ductile behavior. Due to use of shear reinforcements, horizontal and vertical displacement capacities of flat plates under lateral loading were increased compared to those of control specimen.