Theoretical studies are presented on the prediction of bending strength of a long concrete-filled rectangular tube subjected to bi-axial bending. The main objective of this study is to refine the AIJ design formula which is based on the modified superposed strength method. Sophisticated moment magnification factor is introduced, which can be applied to long columns subjected to non-symmetric bi-axial bending. Bending strength of a long column is calculated as the full plastic moment, which is obtained by the generalized-superposed strength method, divided by the moment magnification factor. Proposed formula shows a good agreement with precise numerical solutions.
