A METHOD FOR THE TREATMENT OF 2ND ORDER EFFECTS IN PLASTICALLY-DESIGNED STEEL FRAMES
F. Walport 1*, L. Gardner 1 and D.A. Nethercot 1
1 Imperial College London, London, UK
The susceptibility of steel frames to global second order effects, also referred to as sway effects, ‘P–D’ effects and global geometric nonlinearities, is traditionally assessed through the elastic buckling load factor αcr. For elastic analysis, EN 1993-1-1 and other international steel design standards deem second order effects sufficiently small that they may be ignored if the amplification of internal forces and moments is no more than 10% of the original forces and moments, corresponding to a limit of αcr≥10. For plastic analysis, to reflect the influence of the degradation of material stiffness, a stricter limit of 15 is prescribed for second order effects to be neglected. Use of a single limit of 15 for any structural system no matter the degree of influence of plasticity is considered by the authors to be overly simplistic. A parametric investigation to assess the stability of steel frames in the plastic regime is presented herein. Based on the findings, an improved method is proposed for the treatment of global second order effects in plastically-designed steel frames.
Frame stability; Global analysis; Numerical modelling; Plastic design; Second order effects