Behavior of cross-shaped stiffened concrete-filled steel tubular stub columns after fire exposure

The cross-shaped stiffened concrete-filled steel tubular stub column is constructed by incorporating steel reinforcement ribs into the traditional cross-shaped design, which helps delay the buckling of the steel tube and enhances its restraining effect on the concrete. However, existing standards and research proposed design methods for estimating the post-fire bearing capacity of steel reinforced concrete stub columns are primarily applicable to the ordinary steel tubes with conventional thickness and sections. Therefore, it is necessary to investigate the axial compressive properties of high-strength concrete stub columns with irregular thin-walled steel tubes after exposure to fire, which can contribute to their potential future application. Effects of various experimental parameters, such as heating time, width-thickness ratio, and longitudinal spacing of stiffeners were investigated. In total one specimen subjected to ambient temperature and eight cross-shaped stiffened concrete-filled steel tubular stub columns subjected to fire conditions were prepared for measuring temperature-time curves of specimens during ISO-834 standard fire, load-displacement curves, load-transverse, and longitudinal strain curves of specimens under axial compression loading. The failure modes of the specimens were recorded, and the influence of each parameter on the post-fire mechanical properties of the specimens was analyzed based on the experimental results. ABAQUS software was utilized to develop a model for post-fire analysis, which was validated by comparing it with experimental data. After validation, the model was used to analyze the underlying mechanism, and a comprehensive parametric study of members was conducted. Based on the results of the parametric study and the model developed for calculating bearing capacity of the cross-shaped stiffened concrete-filled steel tube stub column at ambient temperature, a simplified equation accounting for the effects of elevated temperature was proposed to predict the residual bearing capacity of the cross-shaped stiffened concrete-filled steel tube stub column after fire exposure. The average error between the simplified formula and the finite element simulation results is 0.925, and the mean square error is 0.085.