A method for the treatment of second order effects in plastically-designed steel frames

Abstract

The susceptibility of steel frames to global second order effects, also referred to as sway effects, ‘P–Δ’ effects and global geometric nonlinearities, is traditionally assessed through the elastic buckling load amplifier αcr. For elastic analysis, EN 1993-1-1 and other international steel design standards state that second order effects may be neglected provided αcr is greater than or equal to 10. However, when plastic analysis is employed, yielding of the material degrades the stiffness of the structure, and hence a stricter requirement of αcr ≥ 15 is prescribed in EN 1993-1-1 for second order effects to be neglected. Use of a single limit of 15 for any structural system is however considered to be overly simplistic. A more consistent and accurate approach is to determine the degree of stiffness degradation and hence the increased susceptibility to second order effects on a frame-by-frame basis. A parametric analysis to assess the stability of steel frames in the plastic regime is presented herein. A series of frames with varying geometries and load cases has been assessed. Based on the findings, a proposal for the calculation of a modified elastic buckling load factor αcr,mod, which considers the reduction in stiffness following plasticity on a frame-by-frame basis, is presented.