Proposition of isolation system applicable to the long-period earthquake ground motion (Method of changing natural frequency of isolation system with additional spring)

At present, there are many types of isolation system, and many of them are designed for the short-period ground motion. The long-period ground motion has recently attracted the attention, though the isolation system for the short-period ground motion does not have an ability to suppress the vibration due to the long-period ground motion because the frequency of the long-period ground motion is near the natural frequency of the isolation system. In this study, a new isolation system is proposed for not only the short-period ground motion but also the long-period one. The basic concept is that the dynamic property, which is the natural frequency, is changed by adding a spring when the amplitude of isolation table exceeds a threshold value. The validity of the method is checked by the numerical simulation using a single-degree-of-freedom system against the sine sweep ground motion and the actual earthquake ground motion. As a result, it is shown that the resonance can be suppressed by the proposed method. Then the applicability is checked by using the experimental setup. In the experiment, the additional spring is connected with the isolation table by using the clamping mechanism. The adequate clearance between the isolation table and the additional spring is determined in advance. As a result, the system can suppress the resonance phenomenon, so that it was concluded that the proposed system has a sufficient ability to suppress the resonance due to the long-period ground motion.