Post-fire mechanical behaviour of ultra-high-performance concrete-filled steel tube (UHPCFST) stub columns under compression

Twenty-four full-scale tests were carried out in order to investigate the post-fire behaviour of UHPCFST stub columns under axial compression after being exposed to elevated temperatures. The test parameters include cross-sectional size, heating mode, steel fibre content, and coarse aggregate content. The failure modes, cross-sectional historical maximum temperatures, axial load-deformation curves, and residual compressive capacities of the specimens are analysed. The test results revealed non-uniform and nonlinear distributions of the cross-sectional historical-max temperatures. It is found that the effect of historical temperature on the residual capacity of a UHPCFST column is more obvious with the increase of the heating rate and decrease of the cross-sectional size. Inspired by the concept of average temperature method under fire, a method for calculating the post-fire residual load capacity of CFST columns was proposed based on the average historical-max temperature of the cross-section. High accuracy has been found for using the proposed calculation method, therefore the work could serve as a reference for the post-fire assessment and strengthening of UHPCFST stub columns.