A new design method for calculating critical temperatures of restrained steel column in fire

This paper presents a new simplified design method for calculating the buckling- and failure-temperature of uniformly heated, axially restrained steel column subjected to axial compression load or combined axial compression force and bending moment in fire. Here the term “buckling temperature” refers to the temperature at which the restrained column loses temporary stability due to increased axial compression load, while “failure temperature” refers to the temperature at which the axial load in column returns to the initial level. For a restrained column under axial load, this paper presents effects of three different column design parameters, including the axial restraint stiffness, the initial axial load and the column slenderness, on the column buckling- and failure-temperature using a calibrated finite element method model. Two additional parameters are analyzed, including the bending moment ratio and the end moment ratio, for a restrained column under combined axial force and bending moment. To derive design calculation equations, regression analyses were carried out to express the reduction in column buckling- and failure-temperature, from that of the column without axial restraint, as functions of the aforementioned column design parameters. The accuracy of these design equations are then assessed by comparison between their predictions and finite element predictions.