Nonlinear fire analysis of steel structure using equivalent thermal load procedure for thermal geometrical change

Abstract

When a steel structure is subjected to fire attack, the thermal material degradation of the steel members can incur the premature material yielding, and also the thermal axial expansion and bowing on a steel structure can change its structural geometry. These thermal effects lead to the material and geometric nonlinearities of a structure, which therefore defy the accurate behavioural prediction by the general methods of analysis, and thereby impair the structural safety of a steel structure under fire. Unfortunately, the fire load is highly uncertain, which in turn undermine the cost effectiveness and reliability of the fire safety design of a whole structure when using the prescriptive-based fire safety design. To this end, this paper presents the nonlinear fire analysis, in which the geometric and material nonlinearities are with recourse to the higher-order element stiffness formulation and the refined plastic hinge approach, respectively. Specifically, the equivalent thermal load procedure is introduced to determine the thermal expansion effect prior to the fire analysis, which can be then incorporated into the higher-order element formulation. Therefore, the present nonlinear fire analysis can replicate the realistic behaviour, including thermal effects, geometric and material nonlinear effects, of an entire steel structure complying with a realistic fire scenario in the efficacious manner using least number of element(s).