To provide a reinforcement structure which allows a rectangular flat metal plate subjected to in-plane shear to secure a shear yield load and to maintain shear strength under advanced shear deformation after yielding.

A typical reinforcement structure for a rectangular flat metal plate is shown in figure (a). Frames 2 and 3 are attached to the peripheral four sides of the rectangular flat metal plate 1 subjected to in-plane shear for reinforcement. Diagonal reinforcement members are arranged at the four corners of the flat plate slightly away from the peripheral frame members such that no truss force is exerted. Main tensile stress +σ in the flat plate plane is supported and the influence of main compressive stress -σ is blocked, so that a shear yield load is secured and shear strength in the large deformation region is maintained after yielding. The state of the rectangular flat metal plate under advanced shear deformation is schematically shown in figure (b). Main compressive stress vanishes in the flat plate plane so as to form a tension field, where the diagonal reinforcement members uniformly support the main tensile stress +σ. The shear deformation is limited in a cross-shaped region shown by dotted lines, so that a shear yield load is secured and shear strength is stably maintained after yielding.