Equivalent geometric imperfection definition in steel structures sensitive to flexural and/or torsional buckling due to compression

• A procedure is described to assess the imperfection for structures.
• An energy method is proposed to estimate the worst direction of the imperfection.
• A simplified method for nonlinear analysis of imperfect structures is described.

The purpose of this paper is to present a proposal for the design of steel structures sensitive to buckling due to compression in order to fill in the gaps in the current Standard EN 1993-1-1 guidelines for obtaining the magnitude of the geometric equivalent imperfection. The proposal generalizes the approach provided in Clause 5.3.2(11) of EN 1993-1-1 (EC-3) for cases in which a torsional or flexural–torsional buckling mode may occur. The extension of the procedure also allows designers to obtain the magnitude and shape of the imperfection as well as the worst direction of the imperfection due to the external loads applied. It also identifies the cases in which it is necessary to consider the shape of the imperfection given by higher buckling modes.