Behaviour of geometrically imperfect sway steel frames with material nonlinearity exhibiting bifurcational instability

The primary objective of this study is to examine the effect of initial imperfections on the inelastic geometrically nonlinear behavior of sway frames through an extensive series of numerical investigations. The proposed methodology accounts for both geometric and material nonlinearities and incorporates initial geometric imperfections. An elastic-perfectly plastic constitutive model is employed to describe the behavior of structural steel, neglecting strain hardening effects in order to provide a conservative assessment of the structural response. The obtained results indicate that geometric imperfections do not always lead to a reduction in the strength of sway steel frames. Depending on the specific characteristics of the structural system and loading conditions, imperfections may even contribute positively to the overall load-carrying capacity, despite the presence of load imperfections. The magnitude of the initial imperfections is defined according to the provisions of Eurocode 3, while different imperfection orientations are considered to assess their influence on the nonlinear response of the frames. The inelastic analyses demonstrate that the ultimate capacity of sway frames is highly sensitive to the orientation of the initial imperfections. Furthermore, in several cases, Eurocode 3 significantly overestimates the design resistances. However, for specific combinations of loading direction and imperfection pattern, the provisions of Eurocode 3 may result in an overestimation of the actual load-bearing capacity, potentially leading to unconservative design outcomes.