Development of Devices that Suppress "Slow Shaking" of High-rise Buildings

2018.12.21

Hazard and Risk Evaluation Research Division,
Professor Kohju Ikago

When an earthquake occurs, various seismic motions occur such as wiggle shaking and slow shaking. The time associated with a single shake is called a “period,” a sharp shake with a short cycle is referred to as a "short-period seismic motion," and a slow and large fluctuation with a long period is referred to as a “long-period ground motion.” High-rise buildings commonly resonate with long-period ground motion; especially high-rise floors experience significant sway motion*.

In Japan, especially after the Great Hanshin-Awaji Earthquake of 1995, an increased number of buildings were equipped with seismic isolation to mitigate earthquake shaking and prevent damage due to a large earthquake (there are seismic isolation devices also in the IRIDeS building). Dampers are attached to the seismic isolation structure to attenuate the shaking motion during an earthquake. Recently in high-rise buildings the number of different designs to install dampers has increased. However, while current dampers can effectively reduce building damage caused by sharp and short vibrations, they are not effective at preventing long-term shaking in high-rise buildings. If the number of dampers were increased to reduce the amplitude caused by rarely occurring long-period ground motion during earthquakes, there is a possibility of amplifying the shaking caused by frequently occurring short-period seismic waves.

The research team of Prof. Kohju Ikago has been working on the development of a new “tuned viscous mass damper” that protects buildings from long-period ground motion. This type of new damper system can absorb vibrations from a slow shake. The idea of “dynamic mass” used in this damper was proposed in Japan in the 1970s and was applied to the suspension of race cars abroad in the 2000s. However, Prof. Ikago’s group was the first in the world to develop large-sized dampers that can be used to suppress the shaking of large buildings. They succeeded in performing experiments and producing life-size prototype dampers in 2009; in 2013, for the first time, their damping system was installed in a building in Sendai. Since then, they have been working on the development of vibration dampers that are effective at suppressing long-period ground motion.

Prof. Ikago set up a damper, under development, in a university laboratory setting (i.e., a shaking table capable of reproducing the movement of the ground caused by an earthquake), and is conducting experiments to determine how much shaking caused by earthquakes can be suppressed. According to Professor Ikago, “Earthquakes with higher magnitudes easily generate long-period ground motion, and many Japanese cities are on a sedimentary layer where the ground is soft, which easily amplifies long-period motion. Additionally, we have a forest of high-rise buildings and seismic isolation buildings susceptible to long-period ground motion; due to these conditions in Japan, the significance of developing effective dampers to respond to long-period ground motion is crucial.” In the future, he anticipates that it will be possible to develop dampers capable of coping with both short-period ground motion and long-period ground motion.

According to Prof. Ikago, Japan is ideally situated to develop effective vibration dampers; furthermore, excellent products have been developed and accepted by society, which enhances their adoption for practical use. However, Japanese dampers are highly precise but expensive by global standards; therefore, their use in building designs can be a difficult sell. Furthermore, according to Prof. Ikago, “the expectation that vibration damping devices can handle all earthquakes is inaccurate.” A normal damper is only effective for horizontal sway, and it does not mitigate vertical shaking. Moreover, any damper cannot tackle cases where the ground itself is displaced by an inland earthquake; therefore, the only possible countermeasure, in such cases, is to avoid building structures on or near active fault zones.

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