Punching and Post-Punching Shear Behaviour of Interior RC Slab-Column Joints with Inplane Constraints

Abstract

A reinforced concrete (RC) flat plate is a type of structural system widely used in the construction of about 80-90% of residential high-rise buildings in Australia and similarly overseas. Flat plate construction is however prone to punching shear failure at slab-column joints which may trigger a catastrophic progressive collapse of the entire structure. In a flat plate structure, unbalanced moments at the slab-column joints are unavoidable. These bending moments are commonly caused by asymmetric loading, unequal spans or horizontal forces like wind or earthquake. The existence of unbalanced moments could increase the vulnerability of a flat plate structure to punching shear failure. In addition, the load originally resisted by a potentially damaged column will be transferred to the adjacent joints, at which a large additional shear stress and unbalanced bending moment will be generated, eventually leading to more complex mechanical performance and failure mechanism of the joints. Limited collapse-resistant design guidelines are available for this type of structure. Moreover, its punching shear and post-punching failure mechanisms in the context of a progressive collapse are still in need of in-depth study. To investigate the punching and post-punching shear mechanisms of slab-column joints subjected to concentric loading (with balanced moments), previous static tests on four slab-column joint specimens with slab in-plane restraints have been analysed. The effects of different punching directions (upward punching and downward punching) and embedded beams on the post-punching performance of the joints were studied. Furthermore, a 3D nonlinear finite element modelling approach for concentrically loaded joints was developed using software LS-DYNA and verified against the test results. Based on the numerical study, the contributions of the concrete and reinforcement in resisting the collapse of the slab-column joints were evaluated. To investigate the mechanical behaviour and resistance of slab-column joints subjected to eccentric loading (with unbalanced moments), experimental tests were performed on seven 1/3-scaled interior slab-column joint specimens with appropriate in-plane restraints. In order to achieve different levels of eccentricities of loading, a special device was designed, through which an upward monotonic asymmetric loading scheme could be implemented. The overall load-displacement responses, crack propagations and strain developments were recorded and analysed. The damage and failure modes at different loading stages were examined and the influence of unbalanced moments, slab thickness and reinforcement ratio on the punching and post-punching shear behaviours of the slab-column joints with in-plane restraints were also explored. In addition, the results from eccentric loading tests were compared with those of the concentric loading tests and the differences between the two were highlighted. Numerically, a 3D nonlinear finite element modelling approach was established to investigate the punching and post-punching shear behaviours of eccentrically loaded joint specimens. Appropriate selection of different element types, bond-slip relationships, loadings and boundary conditions and element erosion ensured the accuracy of the proposed model. The displacement-controlled quasi-static upward loading scheme was implemented in the numerical models by applying a thermo elastic material to the elements representing the loading device. The proposed numerical models were validated against experimental results, in terms of overall responses, crack patterns and failure modes. Good agreement was obtained, which confirmed the accuracy and reliability of the proposed models. Moreover, parametric studies were performed to investigate the effect of lateral boundary restraints, different levels of eccentricity and the strengthening methods on the punching and post-punching shear behaviours of the slab-column joints. Finally, to analytically investigate the post-punching responses of the slab-column joints with in-plane restraints, mechanical models and analytical solutions were proposed for the joints subjected to either concentric or eccentric loading. The analytical solutions developed for calculating the post-punching resistance were based on the interactions between the reinforcement and concrete, satisfying the compatibility of deformation and equilibrium relationships. The analytical solutions were validated against all the tested specimens, including concentrically and eccentrically loaded joints. Furthermore, the contribution of the tensile and integrity reinforcement to the overall post-punching capacity were quantified.