This paper deals with the numerical analysis aimed at study the bearing capacity of pinended steel built-up columns under axial compression. Finite element (FE) models were performed for the columns presented in the literature. The main problem discussed in the article is the shape and magnitude of geometric imperfections introduced into the numerical FE model, necessary to obtain the load capacity consistent with the experimental strength tests. Three types of numerical analysis that can be used inAbaqus program to calculate the load bearing capacity have been described. The imperfections possible to introduce for built-up columns were presented and an equivalent imperfection corresponding to both imperfections recommended by Eurocode 3 (global of the entire column and local of the chord) for built-up members was proposed. The results of the geometrically and materially nonlinear static analysis were compared with the calculations according to the code procedures (Eurocode 3 and PN-B- 03200:1990) and the results of experimental tests.

The study analysed a bisymmetric closely-spaced built-up member, pin-supported at both ends. Itwas bipolarly pre-stressed with a displacement (BPCSBM), and loaded with an axial compressive force. Maximum internal gap between the chords was assumed in the section, in which during the stability failure in a classic closely-spaced member, the largest lateral displacements between nodes would potentially occur. As regards the BPCSBM chosen for analysis, the issues of the buckling resistance in the presence of compressive axial load were solved using the energy method, in which the functional minimisation was performed in accordance with the Rayleigh–Ritz algorithm. The problem of BPCSBM stability was also solved using FEM. A spatial shell model was developed. The stability analysis was performed. The analysis resulted in obtaining the buckling load and the member buckling modes. A general conclusion was formulated based on the results obtained: bipolar pre-stressing leads to an increase in buckling resistance of closely-spaced members.

Attractive large-scale glazing is currently an architectural trend. However, achieving adequate stiffness for larger glazing spans requires the use of complex cross-sections, generally aluminium sections of considerable height. Members with openwork webs are sometimes used in order to achieve increased load-bearing capacity and stiffness with reduced weight. The disadvantage is that this solution takes up a lot of space inside the building. A recently patented diagonalless member attempts to solve the above-mentioned problems. The member is fully demountable and allows glass units to be installed in the space between the chords. It consists of two chords spaced apart by metal sleeves with bolts passed through them. In this study, preliminary qualitative experimental tests were carried out to determine the behaviour of the member under load and to identify zones sensitive to local deformation. On this basis, numerical models (bar and 3D solid models, including contact interactions) were created and tested. Subsequently, the optimum sleeve spacing was determined, the effect of rotational and translational stiffness reduction at the nodes was investigated, and stress concentration zones and forms of stability loss were identified. A new form of local loss of stability of the chord facewall was identified, the so-called sliding push effect of the chord walls on the sleeve (within the larger openings). This is a completely different type of chord facewall failure from that found in known tubular welded joints. The research programme focused on identifying the phenomena occurring in the new member in order to provide a basis for further, more advanced analyses.