Simplified analytical solution for axial load capacity of concrete-filled double-skin tube (CFDST) columns subjected to fire

Concrete-filled steel tube (CFST) columns and even more so concrete-filled double-skin tube (CFDST) columns have demonstrated a desirable behavior when subjected to fire loading conditions. Current methods to estimate the axial load capacity for these columns exposed to fire are either limited to simple cases using tables or require complex finite element analysis. In an effort to develop a more general and practical approach to this problem, a simplified step by step analytical procedure is proposed for the calculation of the axial load capacity of CFDST columns subjected to any given time-temperature (fire) curve. The procedure was defined by combining an analytical solution to the heat transfer problem with calculation of axial load capacity using the fire-modified material properties adopted from the Eurocode 4 general rules for structural fire design. The proposed method was verified with existing experimental and advanced finite element simulation results. A number of design recommendations, based on the knowledge gained from using the procedure for different case studies, are proposed for CFDST columns subjected to fire.