Axial–shear–flexure interaction hysteretic model for RC columns under combined actions

This paper proposes a coupled axial–shear–flexure interacting (ASFI) hysteretic model that can capture the nonlinear interactive behavior of RC columns under combined action of shear, bending moment and variable axial load. The model utilizes a novel concept of normalization to parameterize the primary curves at different axial load levels in addition to the consideration of shear–flexure interaction. An axial load independent stress level index is developed and used with a two-step loading approach to enable the transition between different reloading and unloading branches corresponding to variable axial load levels during seismic shakings. The proposed model has been implemented and calibrated in a displacement-based finite element program. It is able to realistically simulate the pinching behavior, strength deterioration and stiffness softening under combined actions. The validation against the experimental results of columns subject to varying axial loads shows very good agreement. Particular factors affecting the ASFI behavior of RC columns, such as the arrival time and amplitude of vertical ground motions, are also evaluated. Finally, the seismic responses of a realistic prototype bridge are conducted to demonstrate the effects of vertical ground motion on the lateral displacement and force demands.