Numerical analysis of axially loaded rectangular concrete-filled steel tubular short columns at elevated temperatures
Elevated temperatures significantly reduce the local buckling strengths of steel tubes and the ultimate strengths of rectangular concrete-filled steel tubular (CFST) columns exposed to fire. No fiber-based models have been developed that include local buckling effects on the fire-resistance of rectangular CFST columns. This paper presents a new fiber element model for the fire-resistance predictions of axially loaded rectangular CFST short columns at elevated temperatures considering local buckling. The thermal analysis problem of a CFST column is solved by the finite difference method to determine the temperature distribution within its cross-section including an air gap, concrete moisture content and the emissivity of exposure surfaces. The nonlinear stress analysis of axially loaded short CFST columns under fire recognizes the stress-strain behavior of concrete and steel at elevated temperatures. The expressions for initial local buckling and effective widths of steel plates are incorporated in the computational model to include the effects of local and post-local buckling on the fire responses of CFST columns. The existing experimental and numerical results are utilized to examine the accuracy of the fiber-based model. The fiber model developed is used to undertake parametric studies on the effects of local buckling, geometric and material properties and loading ratio on the thermal and structural responses of CFST short columns and the load distribution in steel tube and concrete. The numerical model proposed is demonstrated to simulate well the fire and structural performance of axially loaded CFST short columns under fire. Moreover, computational solutions presented provide a better understanding of the thermal and structural responses of CFST columns in fire.