A master thesis about stiffening plates with very slender stiffeners is presented. The master thesis represents a completion of my master studies and is written in order to qualify for the degree of M.Sc., Master of Science in Structural Engineering at Delft University of Technology. Current work is done in close collaboration with Iv-Consult, an “Engineering Company with Passion for Technology” located in Papendrecht, The Netherlands. Its aim is to develop a semi-analytical tool for cost and time efficient design of plates stiffened with arbitrarily positioned very slender stiffeners in order to reduce the weight of structures where this is critical, such as cranes and tilting box girder beams. As plated structures become bigger and bigger, their self-weight reduction becomes more and more important. In order to achieve a maximum level of optimization with respect to self-weight, plates’ thickness is reduced and stiffeners are used to provide them stability. In current design practice, standards codes are used (like EN1993-1-5 ) that are known to be conservative and limited in order to cover all type of cases. In this kind of structures, there are many individual plates and therefore, non-linear FEM analysis of all of them becomes relatively time consuming and requires experienced engineers. Since the level of conservativeness of faster methods is dependent on plate configuration, for some cases, through a non-linear FEM analysis, the strength increase can be significant with respect to EN1993-1-5, while for others it is almost inexistent. The company wants to know what the approximate amount of this conservativeness is for a certain configuration so that it can assess on which cases is time and cost worthy to do a detailed nonlinear FE analysis and on which ones the gain is not worth the cost. In order to achieve this, a design tool is developed, under the name of Iv-Plate, having as foundation a semi-analytical method based on the principle of stationary potential energy combined with numerical solution. A method of reducing the structure’s own weight by using slender webs stiffeners is also analyzed and integrated within the tool.