Multi-surface strength model for concrete beam-column joints subjected to cyclic loading

Laboratory tests of structural elements, in particular the reinforced concrete beam-column joints, subjected to cyclic loading provide useful information of the structural damage and post-damage ductile behaviour during an earthquake. With the advent of computer technology, it is possible to study the complicated phenomena through numerical simulations. However, the major hindrance lies in the establishment of a sound constitutive model for concrete. This paper puts in place a multi-surface strength model for concrete, which accounts for the elastic, plastic, damage and post-damage behaviour. It is a semi-theoretical model, in which the strength envelope is derived from experimental meridians and completed through strength theory. Different from those popular but over-simplistic strength criteria, such as Tresca and Mises, the present strength model takes into account all stresses. Eventually, the strength model is presented in multi-surface form in the 3-dimensional stress space (pi-space) for different phases. The key one is the maximum strength surface, which is subsequently used to derive the elastic-limit surface and series of plastic loading surfaces. In addition, evolution of stress states is governed by known rules for the loading-unloading-reloading processes. In the pre-damage phase, non-associate plasticity and hardening rule are employed to govern the behaviour of concrete. In the post-damage phase, anisotropic damage theory is used to describe the stiffness degradation. The numerical simulation of a beam-column joint is presented and compared with experimental results.