Equivalent geometric imperfections for the design of steel and stainless steel beam-columns by GMNIA

In current structural steel design specifications, the stability of members is typically assessed through the use of buckling curves which implicitly consider the influence of both initial geometric imperfections and residual stresses. An alternative and more direct approach is to perform a second order analysis of the member or structure with all imperfections included. For modelling convenience, i.e. to avoid the explicit modelling of residual stresses, so-called 'equivalent' geometric imperfections are typically utilised, which consider the combined influence of both geometric imperfections and residual stresses through an increased geometric imperfection amplitude. For columns and beams, equivalent geometric imperfections for use in design by geometrically and materially nonlinear analysis with imperfections (GMNIA) have recently been developed. However, for beam-columns, which are subjected to combined compression and bending, there are currently no specific provisions. Therefore, equivalent geometric imperfections for application in the stability design of steel and stainless steel beam-columns by GMNIA are developed herein. The proposals are calibrated against the results of benchmark finite element (FE) simulations performed on a large number of steel and stainless steel I-section or hollow section beam-columns, with or without intermediate lateral restraints, subjected to various ratios of axial compression and major and/or minor bending moments. Proposals for the shapes and amplitudes of equivalent geometric imperfections are developed for application with both eigenmode-affine imperfections e0,mod and the direct modelling of sinusoidal bow imperfections e0,bow. Relative to the traditional Eurocode design calculations, employing the developed equivalent geometric imperfections in GMNIA provides more accurate resistance predictions. The reliability analysis of the design provisions shows that partial safety factors of 1.0 for steel and 1.1 for stainless steel can be safely adopted.