Post-fire performance of axially-loaded rectangular steel tube confined reinforced concrete stub columns

The steel tube confined reinforced concrete (STCRC) column represents an innovative form of steel–concrete composite member, in which the steel tube is principally intended to confine the concrete core rather than to carry axial load directly. The post-fire behaviour of rectangular STCRC columns is complex due to the non-uniform confinement effect across the section, while current knowledge and design approaches remain lacking. To bridge this gap, finite element analysis (FEA) is employed in this study to evaluate the axial compressive performance of rectangular STCRC columns after fire exposure. Seven rectangular STCRC stub specimens with varying heating durations and aspect ratios underwent thermal effects followed by axial compression tests. Key responses including temperature evolution, load–deformation behaviour, failure modes, residual capacity and stiffness were analysed and utilised for calibrating the numerical models. The working mechanism of such columns was examined, with emphasis on confinement effectiveness and sectional axial force distribution. Parametric studies were carried out to quantify the effects of key variables on the temperature field and residual mechanical properties. Results indicate that rectangular STCRC columns with identical cross-sectional areas but different aspect ratios tend to reach similar peak temperatures and retain comparable residual capacities and stiffnesses. Concepts and simplified formulas are presented for the first time to predict the historical maximum temperatures of the steel tube, reinforcement, and concrete for assessing material degradation. Finally, predictive equations for the residual ultimate capacity and axial stiffness of rectangular STCRC columns are recommended based on modifications to ambient-temperature design methods given in current standards.