Experimental Research Of Concrete-Filled Circular Steel Tubular Stub Columns With Initial Stress During The Whole Period Of Fire

At present, the researches on mechanical behaviors of concrete-filled steel tube (CFST) columns after exposure to fire are based on that CFST columns sustain no load when suffering fire(high temperature). Namely, the high temperature experiments of the CFST columns were carried out while they were not load-on. However, it does not accord with the actual service conditions of the concrete-filled steel tube columns during fire. In fact, the CFST columns used in engineering were always bearing some design loads (N0) when the fire came. As the indoor temperature (T) increases, the CFST columns will undergo a complex temperature-load coupling process. However, the study on this aspect is still rarely reported. So the tests on the mechanical behaviors of concrete-filled circular steel tubular stub columns with initial stress after exposure to high temperature were investigated in this paper. The main achievements can be summarized as follows:(1) Research findings on thermal properties of steel and concrete under high temperature are summarized at home and overseas. Based on them, the finite element analytic model was established with the finite element software ABAQUS/Standard6.5 to compute the section temperature field of CFST columns and concrete prism specimens heated by electric furnace. The temperature field distribution rule of specimens under different heating systems was also analyzed.(2) In order to validate the theoretical model of the temperature field distribution, tests of two CFST columns and three concrete prism specimens were conducted on the section temperature field. The experimental results were in good agreement with finite element analysis results and prove the validity of finite element model.(3) In order to examine the axial mechanical behaviors of CFST columns with initial stress after exposure to fire, the tests of sixteen specimens were carried out. In these tests, the destructive pattern and high-temperature cooling effect of these specimens during the whole period of fire were discussed, and the influences of temperature and the axial load ratio on axial stiffness and ultimate bearing capacity were analyzed.(4) For the sake of further theoretically researching on CFST columns under the whole period of fire, some experimental studies on the axial compressive mechanical behaviors of 27 concrete prism specimens under the high temperature-descending branch have been carried out; the relationship between free expansion deformation and temperature under ascending and descending branch was investigated; the variation of axial compressive strength of concrete prism specimens under high temperature-descending branch was also analyzed; finally, the axial compressive strength under high temperature-descending branch has been compared to that under high temperature.