Single and interactive buckling modes for unstiffened thin-walled steel sections in compression

The distinction between distortional and local buckling of stiffened elements of thin-walled sections, of both compression and bending members is clearly enough, and relevant design formulas can be used in such particular cases. For unstiffened elements or for those, which are not stiffened enough, the difference between distortional and local budding modes is not clear at all. Local buckling modes involve the thin walls buckling, while the distortion can be regarded as a "sectional" buckling mode. The authors' opinion is that in case of unstiffened elements distortional buckling occurs only, and not the local one. On the other hand. in order to use correctly the Ayrton-Perry formula for the interaction between sectional and overall modest like in case of local-overall buckling interaction, the "distorted" short member has to be defined. The paper presents the authors' proposal for the "distorted" short member and an approach for implementing this in an interactive formula, based on the ECBL approach. Three classical thin-walled unstiffened sections are examined i.e. (sic) and (sic).