Vertical Vibration Isolation Device Using Constant Load Supporting Mechanism

In Japan, near-fault earthquakes have been recorded frequently in resent years, e.g., Hyogoken-Nambu (1995, M7.3), Tottoriken-Seibu (2000, M7.3), Geiyo (2001, M6.4), Niigataken-Chuetsu (2004, M6.8), Fukuokaken-Seihouoki (2005, M7.0), Notohanto (2007, M6.7), and Niigataken-Chuetuoki (2007, M6.8) earthquakes. With such a background, the demand is increasing for protecting important objects like cultural assets and precision instruments from such earthquakes. As a solution to this demand, the use of seismic isolation devices is on the rise. Seismic isolation devices are usually designed to reduce only horizontal ground motions. On the other hand, near-fault earthquakes often cause strong vertical ground motions. Actually, it was reported that an earthware pot, a national treasure, placed on a horizontal seismic isolation device fell over and was broken into pieces in the Niigataken-Chuetsu (2004) earthquake. After the earthquake, which had strong vertical components, the development of vertical seismic isolation devices attracts more and more attention. Developing vertical isolation devices is difficult because of the existence of gravity. In horizontal seismic isolation, inserting low-stiffness elements, such as rubber bearings or coil springs, at the base of the device is effective. On the other hand, if low-stiffness elements are used directly in vertical seismic isolation devices, an unacceptably large deformation takes place because of gravity. Define gravity acceleration by g, and the natural period of an isolation device by T. Then the vertical deformation of the device, x is expressed as x = g(T/2π) For example, if T = 2 (sec), then x = 99 (cm), and if T = 3 (sec), then x = 224 (cm). Various types of vertical vibration isolation devices have been proposed so far. These devices can be classified mainly into the following 3 groups: (a) active and semi-active devices [1], (b) passive devices with motion transformers [2], and (c) passive devices with Euler springs [3]. The devices in the first group use hydraulic mechanisms or air suspensions. Such devices are usually expensive and occupy large space. The devices in the second group, which have mechanisms for changing the direction of motion, do not have such problem. The natural periods of such devices, however, are limited within 2 seconds, as far as experimentally verified, although it is desirable to have more than 2 seconds of natural period to gain enough reduction of response acceleration. The devices in the third group achieved the natural period longer than 3 seconds. Nevertheless, since the devices with Euler springs uses elastic buckling of bars, it is very difficult, if not impossible, to achieve a long stroke, e.g., several to several tens cm, which is essential in seismic isolation. Tt is therefore, strongly desired to develop a passive vertical seismic isolation device whose natural period and stroke are long enough. In this paper, we present a vertical vibration isolation device using a combination of constant-load supporting springs, and demonstrate the effectiveness of the present device through experiments and numerical analyses. Chapter 2 illustrates the mechanism of the proposed device. Chapter 3 reports the results of static loading experiments and shaking table experiments. Chapter 4 describes the accuracy of the prediction obtained by numerical simulations using the Runge-Kutta method.