Residual compressive behaviour of square ultra-high-strength concrete-filled steel tubular stub columns after exposure to high temperatures

Square ultra-high-strength concrete-filled steel tubular (UHSCFST) columns possess high ultimate capacity and stiffness, making them attractive for high-rise buildings. Although severe spalling is frequently observed in ultra-high-strength concrete (UHSC) at elevated temperatures, its influence on the residual mechanical behavior of square UHSCFST column remains insufficiently understood. In this study, the axial compressive behavior of twenty square UHSCFST stub columns (133 MPa of concrete cubic strength) was investigated experimentally and numerically after exposure to temperature ranging from 20 to 800 ℃. The effects of tube wall thickness on core concrete spalling, failure mode, residual ultimate capacity and axial stiffness were also systematically evaluated. The results show that, spalling of the UHSC core initiates at 400 °C and worsens with increasing temperature and decreasing tube wall thickness. Both axial compressive stiffness and residual ultimate capacity decrease monotonically with rising temperature, except for a slight stiffness gain at 200 °C in thick walled specimens. Compared with conventional concrete filled steel tubular (CFST) columns filled with normal strength concrete, stiffness degradation of UHSCFST stub column becomes more severe above 400 °C, whereas capacity degradation becomes significantly worse only at 800 °C. Parametric studies reveal that increasing temperature, exposure duration and concrete strength accelerate capacity degradation, whereas larger sectional width and greater tube wall thickness mitigate it. Available design method is also assessed for their reliability at predicting the residual ultimate capacity, and Han’s and Kamil’s methods using the average historical maximum temperature are recommended for predicting the residual ultimate capacity of square UHSCFST stub columns after fire.