In this paper the Overall Imperfection Method, which is a well validated method for the assessment of the global stability resistance of thin-walled steel structural members with any load and supporting conditions, is used for fire design situation. The method uses equivalent initial geometrical imperfection in the shape of relevant elastic global buckling mode. The assessment of the global stability resistance is performed by geometrically nonlinear analysis and by checking of cross-sectional resistance using reduced elastic moduli in the analysis and reduced design strength for the cross-section checking. The validation of the method is carried out through the analysis of more thousands different structural members with hot-rolled I cross-sections. The reference values for the safety study are calculated by geometrically and materially nonlinear analysis with imperfections.

This paper presents an investigation on the influence of structural imperfections on the ultimate load capacity of steel welded beam-columns with class 4 cross-section under elevated temperatures. This is done by considering different amplitudes for the global and local (plate) imperfections, and different residual stresses distributions available in the literature. To this purpose, a geometrically and materially non-linear finite element model using Abaqus software has been used to determine the buckling resistance of a steel welded beam-column at elevated temperatures, using the material properties of EN1993-1-2. The imperfection sensitivity of beam-columns is reported: the influences of the amplitudes of the geometric imperfection and the patterns of the residual stress on the load capacity are compared.

In this paper, a numerical investigation on the global buckling capacity of the axially compressed steel columns with hot-rolled I cross-section at elevated temperatures is presented. Geometrically and materially non-linear finite element model and the ABAQUS software were used to determine the buckling resistance. The numerical ABAQUS model was validated using experimental results available in the literature, and then the validated numerical model was used to generate a database of load-carrying capacity. The parametric study covered three different cross-section classes (class 1, 2 and 3), ten different non-dimensional slenderness ̄λ = 0.5, 0.6, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9, 2.0), three different temperatures (400°C, 500°C, 600°C), and two stress-strain constitutive relations including (the nonlinear material model adopted in the European guidance for structural fire design EN1993-1-2, and a Bilinear material model), with and without residual stress. The influence of the model parameters on the load capacity of steel columns at elevated temperatures was evaluated. The results of the parametric study were compared with the results of the simplified calculation model presented in EN1993-1-2.