Effects of heating and loading histories on post-fire cooling behaviour of concrete-filled steel tubular columns

A finite element method (FEM) program is developed and used in this paper to analyse the behaviour of concrete-filled steel tubular (CFST) columns during the entire stage of fire exposure, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The emphasis of this paper is on CFST column behaviours during the cooling and post-fire stages because these behaviours are affected by the loading and heating histories, but they have not previously been studied. This paper will present the mechanical property models for these different loading and heating stages. To validate the FEM program, some experimental data, including fire resistance, axial deformation and ultimate strength of CFST columns are compared and it is found that the FEM program can predict the test results with good accuracy. Using the FEM program, a parametric study is then conducted to investigate the influences of ambient temperature loading and heating history on the cooling and post-fire behaviours of CFST columns. It is concluded that various parameters (such as load ratio and elevating temperature time ratio etc.) affect the residual strength of CFST columns severely. Finally, this paper proposes a set of formulas which can be used to predict the residual strength of CFST columns after going through the whole fire exposure process.