Post-buckling of a stiffener beam subjected to single shear load was experimentally investigated. The results were analysed using Wagner theory and non-linear finite elements. A new method was proposed to study the postbuckling behaviour. Each panel is divided into two zones. A central zone where an orthotropic material with a damage law is used and a surrounding zone with the initial material. The results obtained with this analysis were found to agree well the experimental results. The method is less time consuming than classical non-linear finite elements so it can be used for studying complex structures in optimisation procedures.