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The paper presents the development of a new method for the decomposition of any deformed shape of a buckled thin-walled structural member into the contributions of a number of predefined classes of modes. In this method, these classes encompass five types, namely the local, distortional, global, transverse extension and shear modes. The modal decomposition process involves two steps. First, the transverse extension modes are separated out by enforcing the null-transverse-stress criterion. Second, the basis vectors of the local, distortional, global and shear spaces are obtained by either minimizing or maximizing the contributions of the bending energy and membrane energy in the total strain energy.