Experimental study and proposal of a design model of ultra-high performance concrete filled steel tube columns subjected to fire

To contribute a better understanding of the behaviors of ultra-high performance concrete filled steel tube (UHPCFST) under fire, fire resistance tests on twelve full-scale circular columns were performed in this study, with the emphasis on the evaluation of the effects of steel fiber content, coarse aggregate content, and load level. The failure mode, temperature distribution, axial deformation, and most importantly, the fire resistance of UHPCFST columns subject to the thermal-mechanical coupling action were analyzed. The test results showed that irrespective of the mix proportion of UHPC and the level of applied load, all the columns fail in the overall instability. Although the inclusion of steel fiber and coarse aggregate facilitates the heat transferring consider-ably, the temperature within the UHPC core is mostly below 400 degrees C during the whole heating process because of the low thermal conductivity. Moreover, as the content of steel fiber and coarse aggregate increases, better fire resistance of the UHPCFST column is generally observed, benefiting from the enhancement in global stability and the further hydration below the temperature of 400 degrees C. Finally, given the insufficient predictions of the fire resistance of UHPCFST columns upon the thermal parameters and the high-temperature reduction factors ob-tained from existing codes and research works, tailored modifications to the average temperature-based model proposed by the author previously were made. The satisfactory validation with the test results using material high-temperature reduction factors from independent research demonstrated the wide applicability of the pro-posed model.