Equivalent sway imperfections for use in structural steel design by GNIA

Typically, sway imperfections refer to the out-of-plumbness of structural frames and are a key modelling requirement in design by geometrically nonlinear analyses with imperfections (GNIA). Equivalent geometric imperfections, accounting for the combined influence of initial geometric imperfections, the spread of plasticity and residual stresses, are used for modelling convenience. In EN 1993-1-1, the equivalent member bow imperfection amplitudes for use in design by GNIA ${\text{e}}_{0,\mathrm{GNIA}}$ are dependent on the imperfection factor $\alpha$ (i.e. the assigned buckling curve), the material parameter $\epsilon$, the axis of buckling and the type of cross-section resistance check to be performed. By contrast, for equivalent sway imperfections ${\text{ϕ}}_{0,\mathrm{GNIA}}$, two constant values – one for elastic and one for plastic cross-section checks – are provided. The dependency of ${\text{e}}_{0,\mathrm{GNIA}}$ on the imperfection factor $\alpha$ and the material parameter $\epsilon$ rightly reflects the varying influence of residual stresses and material grade on member buckling. However, sway buckling is also influenced by residual stresses and material grade; the suitability of the current EN 1993-1-1 provisions for ${\text{ϕ}}_{0,\mathrm{GNIA}}$ is therefore questioned. In the present paper, new equivalent sway imperfections suitable for use in the design of steel and stainless steel structures by GNIA ${\text{ϕ}}_{0,\mathrm{GNIA}}$ are proposed. The new proposals are shown to be able to provide improved capacity predictions, particularly for stainless steel structures, and are recommended for inclusion into future revisions of Eurocode 3.