In the series studies with the same subject, several kinds of elasto-plastic problems have been treated hitherto. One of them was the plastic strength of framed structures, especially structures subjected to combined loading such as bending and torsion, or biaxial bendings and torsion. In this paper, as one of the examples of such structures, bipod-type masts were analyzed by using matrix displacement method. Although only the collapse loads and mechanisms of the masts were calculated by using upper bound theorem in the previous studies, in this paper successive formation of plastic hinges from the first hinge to the last one was traced and the complete load-displacement curves wered obtained. The numerical results were compared with the previous experimental and theoretical results and showed a satisfactory agreement with them. This method may easily be extended to biaxial bendings and torsion problems in the same way.