Unloading and Reloading Stress–Strain Model for Confined Concrete

Current engineering practice evaluates the seismic performance of bridges based on nonlinear dynamic response analyses. Although various models exist for envelope curves of concrete confined by transverse reinforcement, only a few simple models represent the hysteretic behavior of the confined concrete; therefore, development of an accurate stress–strain model of unloading and reloading paths for confined concrete is urgently needed. In this paper, an unloading and reloading stress–strain model for concrete confined by transverse reinforcement is presented, which is based on the results from a series of compressive loading tests of reinforced concrete column specimens. The model focuses on the effect of repeated unloading/reloading cycles and partial loading excursions, and provides equations that take into account the number of unloading/reloading cycles to predict essential parameters, such as plastic strain and unloaded stress after repeated loading cycles. The proposed model provides good agreement with the test results.