Behaviours of stiffened concrete-filled thin-walled square steel tubular stub columns after non-uniform fire exposure

Behaviours of stiffened concrete-filled thin-walled square steel tubular stub columns after non-uniform fire exposure was studied experimentally and numerically. The considered parameters were heating time, fire boundaries and stiffeners. The failure modes, temperatures in specimens, axial load-displacement curves and strains in steel tubes were measured and discussed. The test results show that all the specimens exhibited waist drum type failures and the concrete becomes more brittle and suffers more severe damage with increased fire exposure sides. The residual bearing capacity, compressive stiffness and ductility index decrease with an increase in heating time and fire exposure sides. Stiffeners can effectively improve the ductility of thin-walled concrete-filled square steel tubular stub columns after fire exposure. A finite element (FE) model was developed using the sequentially coupled thermal-stress analysis method and was validated against tests in this study. Parametric analysis was conducted to identify the influences of key parameters on load-bearing capacity. The FE results show that fire boundary exposure has a significant influence on the residual bearing capacity. The more sides exposed to fire, the lower is the residual bearing capacity. The yield strength and diameter of the stiffeners have no obvious influence on the residual bearing capacity of the specimen. Finally, simplified methods were proposed for predicting the ultimate bearing capacity of stiffened concrete-filled thin-walled square steel tubular stub columns after uniform and non-uniform fire exposure.