Fire design of steel columns through second-order inelastic analysis with strain limits

Abstract

A structural steel fire design approach through second-order inelastic analysis with strain limits is proposed and applied to the fire design of steel columns as a first step in the establishment of a novel fire design framework for steel structures in this paper. The proposed method is carried out using beam finite elements, utilising their computational efficiency. In the proposed design approach, the strength and stiffness deterioration of steel in fire, the spread of plasticity, global instability effects, indirect fire actions and thermal expansion are fully taken into account through second-order inelastic analysis, while strain limits are employed to consider the deleterious influence of local buckling on the ultimate resistance. Ultimate capacity of a steel member or system is determined by (i) the load or temperature level at which the predefined strain limit is attained or (ii) the peak load or critical temperature observed during the analysis, whichever occurs first. A systematic numerical parametric study is carried out through nonlinear shell finite element modelling, taking into account a high number of I-section and hollow section steel columns whose response is considered (i) using isothermal and anisothermal analysis techniques and (ii) with and without axial and rotational end-restraints. It is demonstrated that the proposed fire design approach consistently furnishes significantly more accurate capacity and limit temperature predictions for steel columns in fire relative to EN 1993-1-2 [1] design provisions.