Effect of Torsion on Limiting Temperature of Steel Structures in Fire

Abstract

In structural design of steel structures in fire, the failure criterion needs to be checked for all members under individual design actions or a combination of them. For space frames, combined design actions usually include forces due to bending, axial, shear, and torsion. An increase in temperature will result in changes to these forces as well as the diminution of the failure criterion due to reduced member capacities as a result of these combined design actions. This paper presents an elastic method for the prediction of limiting temperature of members in space steel frames. Since the critical temperature of the structure corresponds to the ultimate failure of the member with the lowest limiting temperature, this elastic method is equivalent to finding the location of the first plastic hinge in a general plastic analysis. By considering the degradation of the mechanical properties of steel in fire, a formulation based on the failure criterion of combined design actions including torsion is presented for limiting temperature calculation. The fundamentals of the solution strategy are also explained.