Abstract
Based on the plastic theory, the author led the equation giving an yield surface of a story of an RC building by using the direction of the deformation of the story as the .parameter for the equation, where the yield surfaces of the members were elliptic, the deformations of the members obey the flow rule associated with their own yield surfaces, and the story have two degrees of freedom in horizontal direction. The equation was analyzed functionally, and general characteristics of the yield surface of the story, such as the shape of them and the relation between them and the deformation of the story, were studied. Through the study, the flow rule associated with the yield surface of the story, and convexity of the yield surface of the story, were proved.
