High-strength Concrete Frame Joints Nonlinear Finite Element Analysis

In this paper, a two dimensional nonlinear finite element analysis of high strength concrete beam-column joints under monotonic and cyclic reversed loading is studied. First, the previous works of nonlinear finite element analysis of high strength concrete beam-column joints under monotonic and cyclic reversed loading is reviewed. The incremental orthotropic model based on the equivalent uniaxial strain, proposed by Darwin and Pecknold, and the Kupfer's failure criterion for concrete is used. The uniaxial stress-strain relationship of high strength concrete proposed by Prof. GUO Zhenghai and postgraduate student XU Jinfeng of Tsing Hua Univ. are used and improved. Based on the experiments in cyclic compression and in cyclic tension done by Reinhardt etc, a modified model of stress---strain relationship for concrete and the method used to solve the problem about the rotation of principal axes in concrete are proposed and modified accordingly The model of stress---strain relationship for reinforcement considering Baushinger effect cyclic reversed loading is chosen, while the ideal elastic-plastic model under monotonic loading is adopted. In the model of bond -slip relationship, a two vertical spring element and inclined struct element are used, bond-slip relationship suggested by Nilson under monotonic loading, while and the model of bond-slip relationship considering bond deterioration under cyclic reversed loading are adopted. A method is used to solve the problem of morbid matrix caused by negative stiffness. Finally, a two-dimensional nonlinear finite element program for high strength concrete beam-column joints under monotonic and cyclic reversed loading is developed. Four high strength concrete beam-column joints are computed and analyzed and some comparison was done between results of experiment and computation. A better model of stress---strain relationship for high strength concrete is chosen. Strength ductility of corner joints are studied. According to the analysis, the calculated and measured yielded load . limited load and hysteresis loop is shown to be in good agreement.