Few people in Tokyo can top the stories of Masato Yamada’s subordinates about where they were when the Great East Japan Earthquake struck last year.
When the ground started shaking at 2:46 p.m. on March 11, Yamada’s workers were 497 meters above ground trying to complete an important but very difficult task on Tokyo Skytree.
The temblor caused the top of the tower--at a height of 625 meters at the time--to sway an estimated 4 to 6 meters. The workers grabbed onto the steel framework, and as materials creaked due to the shaking, they used the emergency stairway to evacuate to the observation deck at 450 meters.
Yamada, head of the tower construction work office for Obayashi Corp., was in an office building under construction to the east of Tokyo Skytree when the quake hit. Yamada, 51, ran toward the nearest window and used a personal handyphone system to contact his subordinates at the operation base in the tower.
Yamada was told all the workers were safe. One of his subordinates then said: "We are in the middle of lifting up the antenna tower. We want to finish the task of lifting it another 20 centimeters."
Broadcasting antennas are attached to the antenna tower, which is positioned at the highest point on Tokyo Skytree. The antenna tower itself is about 240 meters long and weighs about 3 tons, making it the same height as the Roppongi Hills building and the same weight as the Nagoya TV Tower.
When the magnitude-9.0 quake struck, the workers were in the middle of one of the most difficult jobs of the entire construction project--gradually lifting that gigantic antenna tower up through the cavity of Tokyo Skytree. At that time, the antenna tower was being lifted by 348 cables, and some of the jacks to prevent toppling had been removed.
Unless the antenna tower was lifted another 20 cm, all of the jacks could not be put back in place, leaving Tokyo Skytree in an extremely unstable condition.
Yamada did not hesitate in allowing his workers to continue their task. No one knew when the aftershocks would hit, and Yamada gave the simple instruction to his subordinates to watch their footing.
Work resumed after about an hour or two, and the lifting of the antenna tower was completed in about 10 minutes. Workers then positioned all 42 jacks. When the work was done, darkness had fallen and traffic jams had formed hundreds of meters below.
The latest anti-quake technology has been incorporated into Tokyo Skytree, which opens to the public on May 22.
It is designed to escape damage in an earthquake similar to the 1923 Great Kanto Earthquake. It is also supposed to remain standing even if a quake directly under Tokyo is caused by an unknown fault.
Consideration has also been given to winds as strong as 396 kph at the highest point of the structure. Such a typhoon is forecast to hit once in 2,000 years.
Atsuo Konishi, a senior structural engineer in the structural design section at Nikken Sekkei Ltd., also realized that using only established anti-quake or quake-reduction measures would be insufficient for Tokyo Skytree.
"We felt there would be a need for drastic measures to limit the swaying," Konishi said.
The technology that was developed was named "shinbashira" for the central pillar found in traditional five-story pagodas, such as the ones at the Horyuji temple in Nara and the Nikko Toshogu shrine in Tochigi Prefecture. The name was chosen out of respect for the fact that very few pagodas have ever been toppled due to quakes.
The modern version of the shinbashira involved the construction of a steel-reinforced concrete cylindrical structure that was 8 meters in diameter and 375 meters long in the center of the overall tower. The upper part of the shinbashira is not fixed to the overall tower, allowing the two parts to move in different directions during quakes and strong winds.
The shinbashira, connected to the tower by oil dampers that function as a cushion around the structure, is designed to decrease by up to half the swaying from a quake.
In explaining why the term was used for Tokyo Skytree, Konishi said: "The Japanese have a culture of calling the central pillar of a building the shinbashira. We might not have come up with the idea if there were no five-story pagodas."
When the Great East Japan Earthquake struck, Konishi was in the same building as Yamada.
Konishi has had a habit of mentally counting how much time elapses after a quake strikes since experiencing the Great Hanshin Earthquake in 1995. Konishi, who lived in Kobe at the time, knew that if he counted past 30 seconds, there was a high possibility of a severe quake.
The swaying on March 11 last year did not stop for more than two minutes. He stood by a window and gazed at Tokyo Skytree, which remained standing straight.
When the quake struck, the shinbashira did not fulfill its function because it was still attached to the inner wall of the tower.
"We prepared for not only the safety of the structure after it was completed, but also so there would be no problems at each step of the construction process--even if a quake of an intensity of upper 5 struck," Konishi said.
On March 18, 2011, one week after the Great East Japan Earthquake, Tokyo Skytree reached its planned height of 634 meters.
(This article was written by Kenji Katayama, Kazuhisa Kurokawa and Hirotaka Kawakami.)
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