Prediction of buckling strength of stiffened plates by use of the optimum eccentricity method

The buckling capacity of axially loaded stiffened plates, as found in bridges and marine structures, are often analysed by so-called strut models, ref e.g. Galambos (1987). The strut model is traditionally taken as a single span column with hinged end connections. However, the stiffeners in a real structure are continuos over several spans or to some degree rotational fixed at the ends. Therefore, a new strut model is developed. This new concept is called the optimum eccentricity method. This paper presents the background for the method of analysis and the capacity formulations. Furthermore, the present method is compared with numerical calculations made with non-linear finite element methods as well as full-scale tests. The new formulation shows improved predictability in most cases compared to current codes. The method is valid for stiffened plates loaded with axial compression stress both longitudinal as well as transverse to the stiffener combined with in plane shear and lateral load on the plate. The validation of the method presented in this paper is limited to longitudinal compression stress combined with lateral pressure acting both on the stiffener as well as on the plate side. The new formulation for calculation of buckling strength of stiffened panels is implemented in the Norsok standard N-004 for Design of Steel structures, which is used for design of offshore structures in Norway. Whilst the capacity checks are developed for stiffened plates the same method may be utilised for check of beam-columns loaded by axial force and lateral load with buckling about the same axis as the moment caused by the lateral load.