Criteria for estimating the ultimate strength, using a semi-analytical approach, of unstiffened and stiffened plates with a free edge are addressed. The semi-analytical approach is based on a Rayleigh-Ritz discretisation of the deflections, and the equilibrium equations are solved using an incremental perturbation technique with arc length control. Large deflection theory is used to include non-linear geometrical effects associated with slender cases. Development of ultimate strength criteria, accounting for plasticity, is an important part of the present work. The model is implemented in FORTRAN, and the ultimate strength estimates are compared to the strength found with ABAQUS. Testing of stiffened plates are performed for eccentric flat bar stiffeners parallel to the free edge. It is possible to include other types of stiffeners and stiffener orientations in the model. The first linear elastic buckling mode is used as imperfection when testing the criteria. Combined imperfections are also studied for the stiffened cases. Strength estimates produced with the final model are satisfactory when compared to ABAQUS results. These tests are performed for a wide range of plate cases. All important buckling modes for stiffened plates are then considered, except from local buckling of the stiffeners themselves. The latter is normally not a restriction, since design codes requires rather stocky stiffener sections.
Some model assumptions and principles with respect to boundary conditions, permanent plastic deformations and assumed imperfections were also investigated. These effects were studied using finite element analyses, for both unstiffened and stiffened plates. The tests were carried out for a small number of plate combinations, but the results presented indicated that the assumptions made in the semi-analytical model were reasonable.