Uniaxial material model incorporating buckling for reinforcing bars in concrete structures subjected to seismic loads

For reinforced concrete (RC) structures subjected to seismic loads, buckling of longitudinal reinforcing bar is a significant damage stage. The objective of this research is to identify the critical parameters that control the buckling of reinforcing bars in typical circular and rectangular RC columns subjected to cyclic lateral loads. A series of nonlinear finite element simulations of laterally loaded RC columns are carried out to simulate reinforcing bar buckling behavior. Next, to achieve computational efficiency and enable a large range of simulations for a parametric study, a simplified "bar-with-springs" model is developed wherein the longitudinal reinforcing bar in a column is isolated and simulated as a flexural member. The combined restraining mechanism affected by transverse reinforcing and column size is represented by springs at the location of each transverse bar. The simplified "bar-with-springs" model is validated through comparison of average stress-strain curves with high-fidelity "full column" finite element models.;After validation of the proposed model, which is capable of providing with "average stress-strain" response of longitudinal bars, a comprehensive parametric study is performed to identify the effects of several important column parameters on the buckling behavior of the longitudinal reinforcement in RC columns. Features of average stress-strain curves of compressively loaded reinforcing bars are summarized and an equivalent material model is developed for reinforcing steel in RC columns that implicitly incorporates the degrading effects of bar buckling. Constitutive relations as functions of critical column parameters are developed for the direct use in nonlinear structural analysis of RC structures. Comparison of the numerically simulated global response of experimentally tested RC columns confirm the value and significance of the proposed equivalent material models as a simple and effective way to include bar buckling effects in finite element analysis of RC structures.