Base Isolator of Vertical Seismic Vibration Using a Negative Stiffness Mechanism

A model of a base isolator for vertical seismic vibration is considered. It consists of a mass and two springs, one of which is a linear type and the other is of varying stiffness. It is assumed that the varying stiffness spring can be changed from positive to negative stiffness. The varying stiffness is accomplished by the horizontally placed springs which are also connected the vertical spring by rigid links providing geometric stiffness which is negative due to compression in the spring and the axial stiffness from the inclined links which decrease as their orientation approaches the horizontal direction. The numerical investigation shows that the force induced in the vertical spring becomes constant regardless of the displacement around half of the maximum contraction. The experiment also confirmed this force-displacement relationship. In the range of displacement, the force transmitted to an object supported by the isolator due to the ground motion can be limited. This mechanism offers a possible way to prevent an object from being damaged by excessive force during earthquakes.