An-Najah Staff

Steel beams when exposed to fire develop significant restraint forces and often behave as beam–columns. The response of such restrained steel beams under fire depends on many factors including fire scenario, load level, degree of restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam–columns under realistic fire, load and restraint scenarios. The finite element model is validated against experimental data, and the importance of high-temperature creep on the fire response of steel beam–columns is illustrated. The validated model is used to carry out a set of parametric studies. Results from the parametric studies indicate that fire scenario, load level, degree of end-restraint and high-temperature creep have significant influence on the behavior of beams under fire conditions. The type of fire scenario plays a critical role in determining the fire response of the laterally-unrestrained steel beam within a space subframe. Increased load level leads to higher catenary forces resulting in lower fire resistance. Rotational restraint enhances the fire resistance of a laterally-unrestrained steel beam, while the axial restraint has detrimental effect on fire resistance.

Restrained steel members, when exposed to fire develop significant forces and this transforms the behavior of a beam (or column) into that of a beam–column. The load carrying capacity of such beam–columns is determined through axial and moment capacity curves (P–MP–M curves). Codes and standards recommend the use of uniform average temperature for establishing the P–MP–M curves at elevated temperatures. This assumption, though adequate for cases where temperature in steel is uniform, such as a column exposed to fire from four sides, may not be valid for columns or beams exposed to fire from 1, 2, or 3 sides since significant thermal gradients develop across the section. These thermal gradients can cause severe distortion in the P–MP–M curves and render the capacity curves based on uniform temperature inadequate for evaluating the strength of such beam–columns. In this paper, a simplified approach is proposed for adjusting the uniform temperature plastic P–MP–M curves to account for the shape distortion resulting from fire-induced thermal gradients. The proposed method employs a two-step process in which the cross-sectional steel temperatures are calculated first, and then the distorted P–MP–M diagram is computed by adjusting the P–MP–M diagrams based on a uniform “averaged” temperature. The applicability of the proposed method to a design situation is illustrated through a numerical example. It is demonstrated that the proposed approach is well suited for predicting the capacity of beam–columns that develop thermal gradient under fire.