In general, a seismic load acting on a structure is considered to potentially cause unstable gross plastic deformation that is called as plastic collapse, because the seismic load induce an inertia force on the structure which may act as an external force onto the structure. The past researches by the authors to clarify the characteristic of seismic loads found that the way of the seismic response on the structure is driven by the correlation between the seismic loading and the natural frequency of the structure while only the dynamic loads are acting. On the other hand, existence of relatively large sustained loads such as a dead weight was also recognized to promote an unstable gross plastic deformation in the past experimental studies.

Based on the previous studies, two factors that are the sustained load level and the correlation other than the dynamic load level are expected to play an important role in determining the dynamic behavior of a structure and potentially governing the failure mode. The several elastic-plastic analyses with an elastic-perfect-plastic (EPP) material property and a simplified structure were conducted in this paper by slightly changing the level of the sustained loads and the dynamic loads. From the analytical results focusing on the ratchet deformations and residual deformations, the level of the sustained load on the structure was found to promote ratcheting behavior on the structure with a specific trend and significantly affect the dynamic behavior at the conceptual conditions which were defined and identified in the past researches.

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