## 1 Introduction
In the discussion, the influence of \"complete\" end plates connected to flanges and the web, and also \"incomplete\" end plates connected only to the web was taken into consideration. In study [19], [PERSON] discussed different aspects of lateral torsional buckling of beams that have limited rotational freedom at supports. The author considered a situation in which the bent element is strengthened (\"complete\" end plates used and connected to the structure at the nodes), and the one in which the beam is weakened compared with fork support (e.g., the use of \"incomplete\" end plates welded only to the web, and of coped beam flanges at the support nodes). The author reported an advantageous effect of the additional stiffness of \"complete\" end plates connected to other structural members. However, for connections in which \"incomplete\" end plate is welded only to the web, and at the same time, the section is reduced due to flange coped, lower critical resistance of the beam is found compared with the fork support. That may have a disadvantageous effect on the evaluation of the member resistance, and consequently, on the structural system reliability. In study [29], the impact of \"complete\" end plates on the increase in the critical moment for lateral torsional buckling was considered. Computations for different cases of beams were done analytically and validated using the Finite Element Method (_Abaqus_). In studies [18; 22], the influence of reduced connection of beams (\"incomplete\" end plate welded only to the web and coped flanges of the section) on the critical moment was analysed. It was confirmed that such a structural design options lower the value of the critical load compared with the fork support. In study [28], [PERSON] and [PERSON] determined the critical moments for beam lateral torsional buckling for the case of stiffening against warping with the support ribs with closed circular section. The solution to the problem was obtained for a single-span beam, and a cantilever beam loaded with a constant bending moment over the member length. In such a case, the solution to the differential equation for lateral torsional buckling is known. It was shown that the use of support ribs with closed section significantly increased the critical loading of the beams of under consideration. In study [37], the influence of various types of supporting ribs with a closed cross-section on the critical moment of the lateral torsional buckling for differently loaded beams was examined. Detailed calculations were made for uniformly loaded beams. It was found that from the technical and practical standpoint, ribs made of channels offer the most favourable option as they do not need to be cut along the length as is the case with circular tubes. The impact of different types of stiffeners (ribbings) against warping that are located at the site of beam support, on the value of the critical moment was examined in study [33]. The formulas for flexural stiffness of ribs and for the degree of elastic restraint against warping for a few types of rib arrangements were provided. It was confirmed that the greatest restraint of section warping is ensured by closed ribs that show the highest torsional stiffness. The analytical results were compared with FEM findings (_Abaus_). Study [35] reported theoretical and experimental investigations into beam with cold-bent open sections, to which the transverse load was applied outside the section shear centre. The impact of the partial stiffening against warping in the support sections was analysed. The experimental and theoretical results were compared with FEM data (_Abaus_). [PERSON], e.g., in studies [24; 25], took into account the influence of point bracings located in the plane of lateral torsional buckling, and also the impact of point elastic bracings that confine warping on the value of the critical moment. The author analysed single- and multi-span beams and also and beams with cantilevers. Experimental investigations were conducted for selected arrangements of point elastic bracings.