Postbuckling Behavior of Axially Restrained and Axially Loaded Steel Columns under Fire Conditions

This paper presents results of a theoretical study on the postbuckling behavior of an axially loaded steel column at elevated temperatures whose thermal expansion is restrained by adjacent structural members. This study investigates the effects on the column failure temperature by various factors, including the restraint stiffness during the column loading (expansion) and unloading (contraction) phases, column slenderness, and the initially applied column load ratio. The column failure temperature is defined as the temperature at which the load in the column during the postbuckling phase returns to its initial load. Results of this study indicate that the column failure temperature can be much higher than that of the column at first buckling and the higher the column slenderness, the larger the difference between temperatures of column failure and first buckling. Consequently, the column postbuckling behavior should be considered. Nevertheless, for columns with light restraints (restraint stiffness to column stiffness less than 5%) or high load ratios (load ratio higher than 0.5), the failure temperature of the column with realistic unloading stiffness of the restraint is only slightly affected by the postbuckling behavior.