A unified design method for concrete-filled composite columns under fire exposure
Concrete-filled double steel tubular (CFDST) columns possess higher fire resistance than concrete-filled steel tubular (CFST) and double-skin CFST (DCFST) columns. However, there is a lack of a simple and practical fire design method for such concrete-filled composite (CFC) columns. This paper proposes a unified method for the design of CFC columns exposed to fire, including CFDST, CFST, and DCFST columns. The unified method is developed based on the Rankine approach, in which the temperature-dependent sectional resistance, Euler buckling load, global buckling reduction, and loading eccentricity effect are consistently incorporated. A total of 474 finite element models were developed using Abaqus to simulate the fire behavior of CFDST columns. A new buckling curve for CFDST columns is derived from Rankine's theory. The buckling coefficients are calibrated by the finite element analysis results. A parameter is proposed to account for the influence of loading eccentricity on the column behavior. Furthermore, a MATLAB-based post-processing program is developed to monitor the temperature evolution of CFC columns. The average temperatures are calculated to approximate the temperature fields in a composite column. The accuracy of the proposed unified method is validated by 118 experimental results and 1040 finite element simulations of CFC columns exposed to fire. The unified method is shown to be an accurate and efficient tool for practical design of CFC columns under fire exposure.