The paper describes a combined analysis of both coupled instability (interactive buckling) and post-failure behaviour of thin-walled channel-section beam-column under bending or/and compression. The local and global buckling analysis is carried out on the basis of non-linear stability equations, using the asymptotic Koiter method in the second order approximation and applying compatibility conditions for adjacent beam walls. As the result of the interactive buckling and post-buckling analysis some different modes of failure are predicted and are used as a base for the determination of theoretical models of plastic mechanisms of failure. The analysis of post-failure behaviour is performed using the energy method based on the assumptions of the rigid-plastic theory with additional assumptions which enable to incorporate the strain-hardening into consideration. Solution of the post-failure problem is based on the Principle of Virtual Works. The post-buckling path obtained from the interactive buckling analysis is combined with the failure path gained from the plastic mechanism analysis. A point in the loading path corresponding to a stress state in the boundary layer in the flange under compression which fulfils a yield criterion is aknowledged as a lower bound estimation of the ultimate load. The upper bound estimation of the load-carrying capacity is carried out on the basis of the plastic mechanism analysis. This approach allows the authors to evaluate the structure's behaviour in the whole range of loading: up to and beyond the ultimate load. Numerical calculations for a channel-section beam-column made from structural steel are carried out.
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