## 1 Introduction  
Reducing dead loads and geometric dimensions are essential when designing any load-bearing structure. These criteria are especially true for crane systems, as every weight reduction represents a potential for additional lifting capacity. From a steel structure's point of view, a crane consists of a main girder where the trolley moves with the attached load and the supporting pillars on either side, which transfer the loads to the carriages. An example of a crane built to move wood is shown in Figure 1.  
Typically, the main girder of a crane is constructed as a box girder composed of thin plates. Because thin plates are prone to buckling, the main girder is reinforced with longitudinal and transversal stiffeners. Due to the need for these additional reinforcements, the optimization of weight not only focuses on the optimization of plate thicknesses but also on the positions and number of individual stiffness. Even though such an optimized design requires some effort, knowledge about how a box girder must be designed is available in the industry, given the long and successful history of such box-type main girders. Consequently, new design approaches are sparse, and the room for further improvement on the box girder seems limited. These limitations are why the second author created a novel design for main crane girders, leaving behind the classical box girder approach and developing a completely new cross-section. A comparison of a classic box-type design for a crane with two main girders and the novel built-up section design with only one main girder is shown in Figure 2.  
However, novel and unique compound cross-sections may challenge engineers' performing strength assessments. Global and local buckling are most interesting when the compound cross-sections contain plane or curved thin-walled elements. Therefore, the engineer will seek guidance in the available codes and guidelines, hoping to be equipped with the needed tools to ensure the safety of the resulting product. The stability behavior of beam-type elements can be assessed using the methods given in EN 1993-1-1 [1]. In contrast, the approach to deal with the buckling of plated elements is described in EN 1993-1-5 [2], and shell structures are regulated in EN 1993-1-6 [3]. The stability behavior of beams [4; 5; 6; 7] as well as structures built from curved plates [8; 9; 10; 11; 12; 13] and shells [14; 15; 16; 17; 18; 19; 20; 21; 22] have been the interest of research for quite some time, and the available knowledge is extensive. The top and bottom elements of the new cross-section consist of curved elements, which have been the focus of active research as of late [8; 9; 10; 11; 12; 13]. Some authors have even developed an analytic design approach based on finite element calculations [23]. Unfortunately, this design approach is not directly applicable to the curved elements at hand due to the presumed boundary conditions of the mentioned method. After surveying the literature regarding cases of complex compound cross-sections that combine beam-,  
Figure 1: New design for crane main girders. Figure 2: (**Left**) Classic design of a crane’s main girder for cranes with two main girders; (**Right**) a new design approach for cranes with only one main girder. plate-, and/or shell-type elements, it can be concluded that such cross-sections have been less investigated than the single components that they are composed of. Nevertheless, as far as crane construction is concerned, the analysis presented in [24; 25] shall be mentioned as an example as built-up cross-sections are also analyzed in these investigations. Extensive experimental investigations on large-scale girders do not appear appropriate to establish a design basis for the novel system. The size of the components alone makes it difficult to set up a suitable test rig. Furthermore, the number of specimens required to generate an empirical basis for design assessment would exceed any economic scope.