Pre- and post-punching performances of eccentrically loaded slab-column joints with in-plane restraints

Abstract

In flat plate structures, interior slab-column joints are subjected to in-plane restraints and bending-shear actions, which can significantly impact their mechanical performance to resist progressive collapse. However, research in this area is still lacking. This paper presents experimental and numerical studies of pre- and post-punching performances of eccentrically loaded slab-column joints with in-plane restraints. Quasi-static tests were conducted on seven joint specimens, including one concentrically loaded (CL) joint and six eccentrically loaded (EL) ones. The effects of critical structural parameters (i.e., eccentricity, slab thickness and reinforcement ratio) on the pre- and post-punching performances were investigated by analysing the load–displacement relationships, failure modes and material strains. The experimental results indicated that larger eccentricity could lead to more reduction of the peak punching shear capacity by as much as 62%, whereas it has little effect on the post-punching capacity. In addition, the strain results of the through-column rebars showed that, with the well-restrained boundary conditions, the slab flexural reinforcement (FR) was found to resist the applied load to a large extent at small deformations and still able to contribute greatly during the post-punching stage. Numerical analysis was conducted to gain an in-depth understanding toward the effects of varied levels of in-plane restraints on both slab-column joints and actual flat plate structures. It was found that the lateral stiffnesses in the CL joint model with boundary beams and the substructure model were similar, which was around 400 N/mm. Finally, the interactions between the unbalanced moment and shear force were explored and compared with code predictions (ACI318-19 and EC2-04). Without considering the enhancement effects from in-plane restraints, the codes produced over-conservative predictions. Based on the FE analyses, the correlation between the dimensionless ratio e/d (eccentricity/effective thickness of slab) and punching shear strength was simplified by a combined straight line and a power curve.