Practical rules for defining initial geometric imperfections for in-plane stability design of steel frames

The appropriate modelling of initial imperfections, in terms of both amplitude and shape/direction, is a key challenge in the design of structures by geometrically and materially nonlinear analysis with imperfections (GMNIA). While recent research has addressed the amplitude aspect of imperfection modelling, there remains a lack of clear guidance for determining the appropriate imperfection shape. This is tackled herein through the establishment of comprehensive rules for introducing global sway and member bow imperfections in the in-plane stability design of steel frames. The proposed rules cover the two most commonly used imperfection definition approaches – the direct definition (DD) method and the elastic buckling mode (EBM) method. In the DD method recommended herein, the directions of global sway and member bow imperfections are introduced following a set of prescribed rules. Two EBM methods are recommended. In the first EBM method, the ultimate capacity is taken as the lowest among n values obtained by considering the first n buckling modes as imperfection shapes individually, with n = 6 recommended. In the second EBM method, the bow and sway components of the imperfection shape are determined separately from different LBA in which loads are applied on the key load paths. The bow components are established by combining the first buckling mode for each key vertical and horizontal load path, while the sway component is based on the first mode – provided it is a sway mode – obtained from an LBA considering the maximum gravity load case. The consequences of using no imperfections are also explored. The accuracy of the proposed methods is verified based on a series of moment frames and braced frames considering both regular and irregular configurations.