Design of stainless steel SHS and RHS columns in fire by GMNIA with strain limits

A new fire design method by Geometrically and Materially Nonlinear Analysis with Imperfections (GMNIA) and using strain limits is proposed for stainless steel columns in this paper. In the proposed design method, owing to their computational efficiency, beam finite elements are employed to carry out the GMNIA of stainless steel columns in fire whereby the material strength and stiffness deterioration at elevated temperatures, the spread of plasticity, global instability effects, indirect fire actions and thermal expansion are directly taken into account. Due to the inability of beam finite elements to consider local buckling effects, the use of strain limits determined based on a modified continuous strength method (CSM) is proposed to capture the deleterious influence of the local instability effects on the structural resistances of stainless steel columns in fire. The ultimate capacity of a stainless steel column in fire is determined by (i) the temperature level at which the CSM strain limit is attained or (ii) the critical temperature at which the column is no longer able to carry the applied initial room temperature loading, whichever occurs first. In this study, stainless steel square hollow section (SHS) and rectangular hollow section (RHS) columns with and without axial and/or rotational end-restraints are taken into consideration. The capacity predictions obtained using the proposed design method are extensively verified against the benchmark results from shell finite element modelling and also compared against the predictions from the European structural steel fire design standard EN 1993–1-2.