The thesis deals with compression-loaded, longitudinally stiffened plates with trapezoidal stiffeners of the closed type. It analyses how the methods for determining the elastic critical buckling stress influence the load-bearing capacity of the plate, which is calculated using the effective width method. For the calculation of the load-bearing capacity, both the method outlined in the current standard SIST EN 1993-1-5:2007 as well as the modified method from the upcoming new generation standard SIST EN 1993-1-5:2024 are taken into account. In the first part of the thesis, we compared the elastic critical stresses determined by different methods, both analytical and numerical, for all models considered in the parametric study. The values were compared in relation to the defined parameters of the models. In the second part of the thesis, the effects of the method for determining the elastic critical buckling stress on the bearing capacity calculated using the effective width method from the SIST EN 1993-1-5:2024 standard are analysed. The new generation of the SIST EN 1993-1-5:2024 standard allows the positive effect of the torsional stiffness of closed stiffeners to be taken into account when determining the load-bearing capacity. However, for the determination of the elastic critical buckling stress, it only provides an equation tailored to open stiffeners and recommends the use of numerical software for closed stiffeners. The parametric study also includes an analytical equation for the elastic critical buckling stress that takes into account the torsional stiffness of closed stiffeners. The analyses have shown that the equation in combination with the calculation method from the standard leads to uncertain results when estimating the capacity. In the last part of the paper, a proposal for a correction of the equation is made and the ideas for future work and the development of an equation in a more appropriate form that could be included in future generations of standards are discussed.
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