Punching and post-punching shear failure behaviors of laterally-restrained and eccentrically loaded flat plate slab-column joints with drop panels

Abstract

In concrete flat plate slab-column joints with drop panels, successive punching shear failures often occur at the slab-drop panel interface and along the column perimeter. When such joints are subjected to combined bending-shear actions under an eccentric load, their punching and post-punching shear failure behaviours are further complicated which can significantly impact the progressive collapse behaviour of the structural system. An experimental study was conducted on six laterally-restrained slab-column joint specimens with drop panels. Different levels of load eccentricities and different drop panel thicknesses were considered. Two punching shear failure sections were also identified. For a thick drop panel, the first punching failure took place at the slab-drop panel interface, followed by the second punching shear failure occurring along the column perimeter. For a thin drop panel, on the other hand, punching shear failure only occurred at the column perimeter. The damage patterns, the reinforcement strains and the joint strengths before and after punching failure at two different sections were analysed. Numerical simulation was conducted to investigate the effects of load eccentricity on the inclination of the punching cone. Numerical results showed that the punching cone angles at the slab-drop panel interface and at the column perimeter are, on average, 11.6° and 27.1°, respectively. In addition, larger eccentricity contributed to higher cross-sectional shear stresses on the eccentric loading side, leading to reduced punching shear strength and earlier damage. Furthermore, the experimental and numerical punching shear strengths were compared against the predictions of the Chinese, American and European design codes. Finally, a post-punching strength prediction method was proposed, and the predicted strengths deviated from the experimental results by −16% and 23%, for the thick and thin drop panels, respectively. In addition, the integrity reinforcement was also found to contribute at least 80% to the post-punching shear strength.