The Japan Times – Nov 2, 2016 – A YouTube video of a scene from inside a high-rise in central Tokyo captures the chilling moment a building swaying from the massive earthquake of March 11, 2011. The 51 seconds of footage shows office workers cowering on the 48th floor of the Shinjuku Nomura Building.
Makoto Ikuta, general manager of the office building management department of Nomura Real Estate Development Co., was in the building that day. “It shook for a long time and was the biggest source of fear among the tenants,” he said. The building’s shaking lasted for about 10 minutes.
And now, 5½ years later, the major developer has completed installment of a vibration control system to reduce mechanical vibrations and shorten the duration of shaking in the aging building, which is not equipped with seismic isolators like the modern structures of today.
Vibration control systems were initially developed to diminish the effects of strong winds, and are already used in many Japanese skyscrapers. But the technology, with adjustments, is gaining acceptance as a way to shield older buildings, for which installing seismic insulators would be too costly and time-consuming, against swaying from quakes as well.
“The installed system can reduce shaking by about 20 to 25 percent. The duration of oscillations (which continue even after a quake ends) are shortened by about 50 percent,” Ikuta said of the Shinjuku Nomura Building, which was 33 years old on 3/11.
Yoshifumi Kawanabe, a Nomura engineer, said the version installed in the 210-meter-tall building combines several parts to absorb vibrations in all directions, which change depending on the cycle and tremor level of the earthquake.
The system, a 1,400-ton steel body called a Tuned Mass Damper, or TMD, is mounted on top of the building and moves in opposition to the oscillations of the quake by means of a pendulum, fluids or springs — counterbalancing and absorbing the temblor. It can work without electricity if power has been knocked out.
At a cost of about ¥2 billion, Nomura’s TMD is a simpler system than a TMD installed in the Shinjuku Mitsui Building a year and half ago. It is also a third of the height at just 3 meters.
Nomura’s TMD consists of about 5,800 steel sheets and other parts, which were taken to the top of the building mainly using the building’s own elevators. It took 21 months to get all the sheets, each weighing 400 kg, to the top of the building for installation.
Nomura and Takenaka Corp., a major contractor that joined hands with the developer, jointly filed a patent application in February 2015 for the TMD system.
The government has been monitoring the risks of extended periods of surface shaking from earthquakes.
The long period of ground motion caused by the magnitude-9.0 quake of March 2011, the largest ever recorded in Japan, shook buildings in Osaka Prefecture, about 800 km away from the epicenter, and caused some elevators to automatically shut down, according to the Headquarters for Earthquake Research Promotion under the science ministry.
A potentially catastrophic quake in the Nankai Trough off central and western Japan could cause buildings in the three major metropolitan areas of Tokyo, Nagoya and Osaka to shake for three to seven minutes, according to a report by the Cabinet Office released last December.
The Shinjuku Nomura Building, which accommodates approximately 6,000 people on weekdays, has fulfilled earthquake-resistant standards for a possible Nankai Trough quake that were proposed by the land ministry in June for existing buildings with heights of over 60 meters, Kawanabe said.
There were five earthquakes of 4 or higher on the Japanese seismic intensity scale to 7 in Tokyo’s 23 wards in the six years before the 2011 mega-quake, but there have been 15 since, excluding two large quakes on 3/11, according to data from the Meteorological Agency.
“The thinking is that the aftershocks have continued since (March 2011), although they are gradually dwindling with time,” said Noriko Kamaya, the agency’s senior analyst for earthquake prediction.
Mitsui Fudosan Co. also has enhanced earthquake-resistant functions in its properties “amid a growing concern over safety” and business continuity for tenants after the 2011 quake, according to a statement in May 2015.
The company installed a TMD in the Shinjuku Mitsui Building, adjacent to the Shinjuku Nomura Building, about a year and half before the latter’s installation.
The TMD installed in the 210-meter-tall building was unprecedented in scale, the company says, and can absorb quakes with especially long periods of ground motion. Forty-eight oil dampers, which act as shock absorbers to dissipate energy, were also installed in the lower floors.
SunshineCity Corp., operator of a building complex in Tokyo’s Ikebukuro district, finished installing a TMD system to reinforce the Sunshine 60 skyscraper in August.
The building uses three different types of oil dampers, including a newly developed device that has “double the shock absorber function of conventional oil dampers” at the sides and middle floors of the 240-meter structure, SunshineCity said.
Major construction company Kajima Corp. supplied the systems for the Shinjuku Mitsui and Sunshine skyscrapers, both of which were built around 40 years ago.
Since the March 2011 quake, owners of existing architecture and construction companies have been playing catch-up to improve safety conditions to the level of recently built skyscrapers, which are generally equipped with built-in vibration control systems using oil dampers on each floor.
Of nearly 1,200 buildings 60 meters tall in Tokyo, those built before 1982, the year in which the revised law for enhanced quake-resistant standards on buildings was enacted, total more than 80, according to the metropolitan government.
The two high-rises in Shinjuku and Sunshine 60 in Ikebukuro are the first of the old skyscrapers to acquire the earthquake-resistant technology following a hiatus after the Shinjuku Center Building, built in 1979, was equipped with a vibration control system using many oil dampers on middle floors in 2009.
“Some buildings that are 200 meters tall are concentrated in Tokyo. Our industry, of course, cannot ignore what the land ministry expects as the industry’s voluntary effort,” Nomura engineer Kawanabe said. “Major developers have to care about the competitiveness of buildings they manage,” he said.
In 1923, the Great Kanto Earthquake with a magnitude of 7.9 devastated the capital. “Based on past records, the frequency of quakes on the magnitude-8.0 scale is between 200 and 400 years, but, in the meantime, magnitude-7.0 quakes could occur a number of times,” said Kamaya of the Meteorological Agency.