A consistent approach to the definition of initial geometric imperfections for in-plane stability design of steel moment frames

Geometric imperfections can have a significant influence on the behaviour and ultimate resistance of steel structures. For the purposes of structural design, particularly design by geometrically and materially nonlinear analysis with imperfections (GMNIA), a suitable choice of imperfections, in terms of both amplitude and shape, is therefore required; an inappropriate choice of imperfections can lead to misleading results and unsafe resistance predictions. The development of a practical strategy for defining imperfection shapes in the in-plane GMNIAbased design of steel moment frames is the focus of the present study. The proposals are also suitable for GNIAbased design. Both overall sway imperfections (i.e. frame out-of-plumbness) and individual member bow imperfections, together with combinations thereof (i.e. sway-member imperfection combinations), are considered. Several methods for introducing imperfections are assessed. Each method is categorised depending upon whether the imperfection is established through: (1) direct definition or (2) the scaling of eigenmodes. The recommended direct definition-based method introduces a sway imperfection in sympathy with the applied lateral loading and member imperfections with alternating directions between storeys, as dictated by the column base boundary conditions. The recommended eigenmode-based method uses the first sway mode and determines the number of non-sway modes according to the eigenvalues under the design loading, with all non-sway modes for which the eigenvalues alpha(cr,ns) < 25 also contributing to the imperfection. The eigenmodes are scaled to ensure the bounds of the prescribed imperfection amplitudes are suitably maintained. The consistency and accuracy of the proposed methods are demonstrated for 21 different frame configurations.