The effect of horizontal bi-directional seismic input is not considered in current designing method with equivalent linearizing method, even though there have been cases where a nonlinear response to bi-directional input exceeds the one to one-directional input. The objective of this paper is to grasp the characteristics of nonlinear response of structures to bi-directional seismic input. With an analytical model of a single-mass two-degrees-of-freedom system, we conducted examinations with parameters of mechanical properties: shape of yield surface and yield resistance. It was verified that the response to bi-directional input was often much greater than the one-to-one directional input, and oblateness of the elastic response to bi-directional input was concerned with the characteristics of bi-directional elasto-plastic response.
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