Experimental study on axial mechanical behavior of reinforced concrete-filled square hollow section columns after exposure to high temperatures

In order to research the effects of exposure time and reinforcement ratio on the residual load bearing capacity, stiffness and ductility of reinforced concrete-filled square hollow section (RCFSHS) columns, six specimens at ambient temperature and twelve specimens after exposure to fire were tested. For the tests with 30 minutes and 60 minutes exposure time, temperature distribution in typical sections of columns were measured at their heating stages. The test results show that when compared to the non-exposed tests, after 30 min exposure, for those columns with reinforcement ratios of 1.45% and 3.26%, their residual load bearing capacity, axial compressive stiffness and ductility are respectively reduced by 13.4% and 5.4%, 20.8% and 24.3%, 31.9% and 44.0%. Correspondingly, after 60 min exposure, the residual load bearing capacity, axial compressive stiffness and ductility are reduced by 23.3% and 12.8%, 37.5% and 39.1%, 52.5% and 61.5%, respectively. Based on the experimental statistics and Code GB 50936-2014, a design formula was proposed for predicting the residual axial load bearing capacity of RCFSHS columns after exposure to fire. It is shown that the calculated results predicted by the proposed formula agree well with the experiment results.