Natural frequency automatic variation in seismic isolation system
The proposed system is based on the following operations: 1. interruption of the continuity between the building and the foundation-soil complex; 2. self-centring of the building after an earthquake. By means of its own elastic deformation each bearing can automatically compensate the rigid deflection variation relative to support due to the horizontal component of the motion. The building remains motionless with respect to the foundation-soil complex, which moves. The vertical motion, due to the sub-undulatory shock, varies the building behaviour only partially. It remains stationary with respect to the horizontal translation, but it is subject to a low vertical translation and to a resonance possibility. In order to prevent the resonance danger, the vertical natural frequency variability takes place because of auxiliary springs which automatically increase the action of one or more main springs during an emergency, characterised by an interval of vertical seismic frequencies including the resonance one.
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