Unified cyclic stress–strain model for square and rectangular UHPC-filled stainless-steel tubular columns

Cross-sectional shape has a notable effect on the cyclic axial stress–strain curves of ultrahigh-performance concrete (UHPC)-filled stainless-steel tube (UHPCFSST) columns. Currently, no unified model can describe this influence mathematically. In this study, UHPCFSST samples having two types of cross-sectional shapes i.e. square and rectangular shapes with various dimensions, were designed, and cyclic axial compression tests were conducted. The key investigated mechanical behaviors of UHPCFSSTs include failure modes and cyclic stress–strain relationships. The mechanisms behind the differences in the plastic index and unloading stiffness degradation between square UHPCFSSTs and rectangular UHPCFSSTs were analyzed. Based on the test data, models for plastic strain, new point strain, and return strain for square and rectangular UHPCFSSTs were established. Considering the effects of cross-sectional shape and confinement, the stress–strain models for unloading and reloading paths were proposed. This resulted in the development of a unified cyclic axial compression stress–strain model applicable to UHPCFSSTs with square and rectangular cross-sectional shapes. The proposed model exhibits high accuracy and applicability, effectively quantifying the impact of cross-sectional shapes on the cyclic axial compression stress–strain curves of UHPCFSSTs.