Multi-span steel frame structures are often used in school gymnasiums and factories. When braces are placed on the gable walls and roof surface of the structure, the seismic load is transferred between the frames, and the rigidity and shear loading capacity of the structure against seismic motion is increased. However, the collapse behavior becomes difficult to predict without the three-dimensional nonlinear structural analysis. The purpose of this paper is to predict the ductility index F, the plastic hinge rotation angle and the roof brace plastic modulus without static incremental analysis and eigenvalue analysis for the three-dimensional structure. Only the stiffness and shear loading capacity of each plane gabled frame are used, and the plastic deformations of the three-dimensional structure is predicted by some simple spreadsheet calculations. The results of the proposed method and three-dimensional time history response analysis are compared to confirm the effectiveness of the method.
