The diploma thesis addresses longitudinally stiffened steel plates subjected to uniform compression. Ultimate resistance of such plates is characterised with global and local buckling as they possess substential post-critical resistance after the elastic critical stress has been exceeded. The design rules of SIST-EN 1993-1-5 for the determination of the effective area were discussed. The main focus is set on the computation of the global reduction factor ρ, which is needed to determine the effective area of a plate under compression. For calculation of the global reduction factor ρ, column-like σcr,c and plate-like σcr,p critical buckling stresses need to be determineed. As the equations for the computation
of the resistance of longitudinally stiffened plates given by the Eurocode are very complex, the implementation of modern computer programs such as EBPlate into the given design rules has been studied. With a verified numerical model a parametric geometric and material nonlinear analysis with imperfections was carried out to determine the ultimate resistance. It was concluded that the use of SIST-EN 1993-1-5 expresions for elastic critical stress lead to very conservative results as it does not take into account torsional stiffnes of stiffener, which is greater for closed than for open flat stiffeners. However, the use of EBPlate in combination with design rules given in SIST-EN 1993-1-5 may lead to unsafe results in the case of closed
stiffeners. Therefore, for plates with closed section stiffeners a new buckling curve is introduced in order to obtain numerical observed resistance, if software EBPlate is used to determine elastic critical buckling stresses.