Many earthquakes have occurred in Japan, and Japanese design seismic wave for several plants have became more severe after strong earthquakes. Especially, since vertical seismic conditions have became more severe, it is difficult to ensure the safety margin of the seismic evaluations in industrial facilities. As a way to ensure the margin of seismic evaluations, it is effective to adopt the vertical seismic isolation system. When the vertical frequency with the vertical isolation system becomes low frequency range, the rocking vibration will occur. By using the seismic isolation system of approximately 3 [Hz] in vertical frequency, the response in vertical direction is reduced without large rocking vibration. However, variations of a specification in seismic isolation system may occur in actual isolation device. Therefore, in this paper, the vibration characteristic including the rocking vibration is discussed, and also an analysis method is evaluated in consideration of the variation of vertical stiffness. Then, using the proposed analytical method, the tendencies of the vertical frequency variation and the rocking vibration response are confirmed. The rocking vibration occurs because of the deviation between the center of gravity and center of stiffness. Therefore, it is necessary to control the decentering between the center of gravity and rigidity. However, assuming the number of actual seismic isolation devices applying the structure, eccentricity ratio is expected to be approximately under 0.05. Therefore, the effect of eccentricity ratio on the rocking vibration is considered to be small.
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