Abstract
The Overall Imperfection Method (OIM) is a highly universal and robust method for the assessment of the global stability resistance of steel members with any load and support conditions. The method uses the relevant elastic critical buckling mode shape as equivalent initial geometrical imperfection and gives a universal procedure for the determination of the proper amplitude. This procedure takes into consideration the appropriate buckling curves connected to a classification procedure of the buckling mode, accordingly gives completely consistent results with the reduction factor based design method where the same buckling curves are used. The assessment (final check) of the global stability resistance is performed by second order evaluation of the properly imperfect member, so the OIM directly calculates the geometrical nonlinearities. Therefore the methodology is completely applicable also in fire design situation since this limit state is characterized by the high nonlinearity. The key issue is the determination of the amplitude of the equivalent geometric imperfection. The method follows the same procedure as used for normal temperature but using the special buckling curves specified for the fire design situation. Applying the equivalent geometric imperfection, the second order analysis is performed with the reduced elastic modulus. By this the result directly includes the effect of the significantly larger deformations caused by the elevated temperature. This methodology is completely consistent with the methodology of conventional reduction factor based buckling design, also in case of elevated temperature. Moreover, it can be used for any situations with complex load interactions or irregular support conditions. The paper discusses briefly the background of the OIM in case of fire design. Moreover, the steps of the application will be described, and an illustrative example will be presented in which the OIM result is compared to the result of the EN 1993-1-2 and GMNIA. The largescale validation of the presented method for irregular structural members with elevated temperature is going on, it is out of the scope of this paper.
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