Equivalent global sway imperfections for use in design by GMNIA

Three types of geometric imperfection need to be considered in the design of structural systems: global frame out‐of‐plumbness, also referred to as sway imperfections, member out‐of‐straightness, also known as bow imperfections, and local cross‐section imperfections. For modelling convenience, equivalent geometric imperfections are often employed to account for the combined influence of initial geometric imperfections and residual stresses. For design by geometrically and materially nonlinear analysis with imperfections (GMNIA), amplitudes for equivalent bow imperfections for members are provided in prEN 1993‐1‐14; however, no provisions exist for global sway imperfections. Therefore, equivalent sway imperfections suitable for use in the design of steel structures by GMNIA, expressed by the out‐of‐plumbness Φ_0,GMNIA are established herein. Two proposals for calculating Φ_0,GMNIA are presented: (1) Φ_0,GMNIA = Φ_0,geom + α/100 with Φ_0,geom representing the amplitude of the pure geometric sway imperfection, and (2) Φ_0,GMNIA = α/50. In both proposals, the varying influence of residual stresses from different cross‐section geometries and axes of bending is captured by the imperfection factor α, as defined by the buckling curves in EN 1993‐1‐1 for carbon steel and EN 1993‐1‐4 for stainless steel, respectively.