Modal Decomposition and Behavior of Free Vibration Response With Grounding and Uplift

Abstract eng:
The effect of uplift on earthquake response of buildings had been investigated by nonlinear earthquake response analyses and shaking table tests of small-scale building models. As a result of these studies, it is found that uplift reduces story shear force that causes structural damage. If it is possible to level off earthquake loading acting on a building by allowing uplift, an epoch-making design system that is independent of input earthquake size or its periodic characteristics may be able to be established. However, it has been found that an uplifting building is vibrating randomly due to higher mode vibration. Therefore it is difficult to predict external load distribution and its size acting on the building. This study presents a modal analysis approach applied to vibrating structures in grounding and uplifting phases. By assuming the non-linear term can be treated as an external force, the modal analysis problem can be simplified and it becomes possible to carry out modal decomposition. Based on this assumption, modal decomposition on response of the uplift building could be carried out as "spring pendulum under gravity". The behavior of SDOF spring pendulum which represents a higher mode is very simple and clarified that vibration was simply caused by movement of an equilibrium position accompanying change of gravity value and direction.

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