Imperfection Shape Combination Rules for the Stability Design of Steel Structures

Abstract Geometric and material imperfections are a key modelling requirement in design by advanced analysis and a suitable choice, in terms of both magnitude and shape, is required; an inappropriate choice can lead to misleading results and unsafe resistance predictions. An overview of a recent study on the development of a practical strategy for defining geometric imperfection shapes in the advanced analysis‐based design of in‐plane steel frames – both moment and braced – is the focus of the present study. Both overall sway imperfections (i.e. frame out‐of‐plumbness) and individual member out‐of‐straightness imperfections, together with combinations thereof (i.e. sway‐member imperfection combinations), are considered. The introduction of geometric imperfections through the direct definition of the imperfect geometry, as well as through the scaling of eigenmodes is explored. The proposed methods were evaluated against ‘worst‐case’ benchmark results obtained from an imperfection combination study that exhaustively revealed the most severe imperfection shape. Overall, the presented imperfection definition methods provide accurate and consistent capacity predictions compared with the benchmark results, while remaining practical for implementation in everyday design practice.