## 5 GMNIA
### Imperfection Form  
Defining the correct imperfection pattern is a crucial step in the computation of the buckling load via GMNIA, and several categories of imperfections are defined in the literature, e.g., in [17; 18; 19; 37]. Estimating meaningful imperfection patterns is complex, even for simple structures such as plate elements or cylindrical shells. Choosing a useful imperfection pattern for a built-up section that combines the above structural elements is even more challenging. As already mentioned, this work deals with the scaled test specimen, and no prior information on the existing imperfection shapes and amplitudes was available when this contribution was written. Therefore, the design computations were performed with imperfection patterns based on the literature and engineering judgment. The above-mentioned challenge can be approached in several ways: firstly, one could consider the possible imperfections of the single parts (beams and panels) before joining; or secondly, imperfections on the cross-section could be derived from the manufacturing process, e.g., weld depressions or collapse-affine imperfections of the whole assembly. A third option would be to consider possible imperfections from the single parts before performing welding simulations to obtain a realistic idea of the complete assembly's imperfections. Given that a GMNIA is already quite an effort, the latter approach would not be practical for assembled parts as the overall complexity of such a procedure should also be considered. Therefore, it seemed reasonable to use so-called substitute imperfections at the components or assembly level. I-beams are less challenging than shells if one considers the possible imperfections of the single parts. Typically, the applied imperfections are the scaled eigenmodes of an LBA, where often the first eigenmode is assumed [38] as the most important one. An alternative approach is to use a bow imperfection, for which EN 1993-1-1 [1] provides guidance on the amplitude for lateral and lateral torsional buckling cases. Regarding the panels, the situation is more complex. Here, eigenmode-affine patterns, as well as patterns derived from the collapse behavior of the perfect shell (quasi-collapse-affine) or shapes derived from post-buckling behavior, might lead to the lowest bucklingload. In addition, imperfections resulting from the manufacturing process might have to be considered in a simplified way. An example of such an imperfection is the depression occurring at welds, which was used in the analysis of welded parts in the literature [39; 40; 41]. On the level of the components, bow-type imperfections were considered globally around the weak axis (z-z) on the webs of the beams. The assumption here is that such bow imperfections should stimulate the phenomena of lateral buckling (LBzz) and lateral torsional buckling (LTB). Because the first eigenmode of a beam under pressure force is close to a bow-type imperfection and by considering that eigenmode-affine imperfections are neglected for the beams, only the bow-type imperfection was applied. For the top panel, eigenmode-affine imperfections were applied. As the first 300 eigenmodes obtained by an LBA influenced the panels on the top side only, it might be safe to conclude that these eigenmodes are associated with the panels only and not with the beams or the whole structure and that the panels are indeed the weak part of the assembly with respect to the loading situation investigated. It is noted in passing that the latter statement might only be true as long as geometrical nonlinearity is not considered. Like a cylindrical shell, the eigenmodes showed clustered behavior, meaning several patterns ought to exist at load magnification factors with only a slight difference. On the level of the whole component, depressions near the welds at the bulkheads were also studied.