入力遮断を目指した免震構造における粘性ダンパ機構の設定法

Abstract

We propose the evaluation and design method of a viscos damping mechanism in the passive-type base isolation system aiming at the acceleration reduction. Through the appropriate setting of the stiffness and the damping between the ground and the building, the longer-period and the additional high damping are attained and the approximate input insulation can be achieved. To get the input insulation effect, the analytical model, the base isolation period of 9 seconds, is analyzed. To restrain the displacement of the base isolation story, the high damping factors of 40% is considered. We found that the input insulation effect can be kept favorably by the Maxwell mechanism, named ‘Maxwell model’ here, where a spring and a dashpot are arranged in series. In this paper, we have analyzed the characteristics of ‘Maxwell model’ by theoretical analyses of the transmissibility, Fourier spectrum analyses, and seismic response analyses. The results of this study are summarized as follows: (1)We made it clear from the displacement-acceleration earthquake spectrum that the specifications satisfying the target performance are the natural periods of 6-8 seconds and damping factors of 30-40%. (2)From the analyses of the transmissibility of the single-degree-of-freedom model, the ‘Maxwell model’ can expect the response reduction effect by the smaller transmissibility compared to the dashpot model, named ‘Usual model’ here, above the specific frequency ratio. Its frequency ratio is about 5 times the natural frequency of the ‘Usual model’. (3)From the analyses of the Fourier spectrum ratio of the fifth-degree-of-freedom model, ‘Maxwell model’ can expect the better response reduction effect for the shorter natural period of the building and the longer isolation period, since the natural frequencies of the insulated system tend to increase. (4)’Maxwell model’ indicates the drastic response reduction effect of the 1st floor's maximum acceleration response compared to the ‘Usual model’ for the longer period and the larger damping factors of the base isolation, and the shorter natural period of the building. ‘Maxwell model’ can reduce the maximum acceleration response to 60 % of the response of the ‘Usual model’ on average for the analytical cases of various earthquakes. (5)The maximum isolation displacement response of ‘Maxwell model’ is almost equal to the response of the ‘Usual model’. For the proposed system, the acceleration response can be less than 80cm/s2 and the isolation displacement can be less than 50 cm for extremely severe earthquakes, in the setting condition of long natural period of 7-9 seconds and the high damping ratio of 20-40%.