A unified method for calculating fire resistance of solid and hollow concrete-filled steel tube columns based on average temperature

This paper presents a new method for calculating fire resistance of axially loaded Concrete-Filled Steel Tube (CFST) columns with different section profiles, including circular and polygonal sections that can be solid and hollow. The uniqueness of this new method is that the fire resistance is calculated on the basis of the average temperature of the columns’ cross-sections. This is done by taking the bearing capacity of a CFST column at room temperature as a special case of the bearing capacity of the same column at the start of a fire. The equivalent strength and equivalent elastic modulus in relation to the average temperature of steel and concrete are investigated, and a unified method of calculation is proposed, by which the calculation of fire resistance of a CFST column can be divided into two steps, i.e. (a) calculation of the equivalent strength and elastic modulus of steel and concrete at elevated temperature based on the average temperature, and (b) calculation of fire resistance using the formulas at room temperature by replacing the equivalent material strength and elastic modulus at elevated temperature. The two sets of formulas for calculating fire resistance of CFST columns are given by combining the unified method, respectively, with Eurocode 4 and the authors’ previous work. The proposed formulas and procedure are validated through comparisons with the experimental results of a number of solid and hollow CFST columns with circular and square sections.