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Sunday, May 20, 2012
The sky's the limit
By BRETT BULL
Special to The Japan Times
After the Great East Japan Earthquake on March 11 last year, the performance of the spectacularly tall Tokyo Sky Tree going up in the capital's downtown Sumida Ward became a subject of heightened interest to experts, residents and the general public alike.
Tobu Tower Sky Tree, the corporate owner of the project, reported that no structural damage resulted from the magnitude-9 temblor, whose epicenter was some 350 km to the north off the Pacific coast of Miyagi Prefecture.
Credit for this accomplishment can in part be attributed to temples all the way down in the ancient capital of Kyoto.
To the casual observer, the steel truss exterior of the 41,000-ton Sky Tree appears to gradually taper from its triangular footprint upward to a cylindrical spire pinnacling at 634 meters. Mostly unseen from the outside, however, is an internal reinforced-concrete column.
"The central column is like those used in the five-story pagodas of traditional Japanese architecture," says Tadano Kamei, a senior architect at architectural firm Nikken Sekkei, which was commissioned for the design work. "The outer frame and central pillar are structurally separate."
In the event of an earthquake, the upper part of the core column is designed to function as "a balancing weight," such that when the inner and outer components begin shaking, their relative motions are out of step and oppose one another. A "viscous damping system" similar to shock absorbers, and positioned at various points between the two, dissipates the seismic energy and lessens the swaying. The result is a 50 percent reduction in the structure's overall movement.
Nikken Sekkei claims that not one temple in Japan that utilizes this design concept, known as shimbashira seishin (central column vibration control), has collapsed due to seismic forces.
The arrangement is just one of many special architectural and design features of this record-setting radio and television transmission tower that's scheduled to open to visitors on May 22.
Built by construction company Obayashi, the Sky Tree is the second-tallest structure in the world after the 828-meter Burj Khalifa in Dubai. Last November, Guinness World Records recognized it as the world's tallest freestanding broadcast structure.
For tourists looking to get a bird's-eye view of the metropolis, two observation decks are positioned at 350 and 450 meters from the ground. High-speed elevators, rolling outside the concrete core that houses an emergency staircase of 2,523 steps, zoom up to the lower platform in 50 seconds, and reach the next deck a half-minute later.
Kamei says that the intention from the start, in February 2005, was to create a structure that "transcends time and space" through a design that allows the tower to function not only as a transmitter and an observatory, "but also an everlasting symbol of Tokyo."
Such a lofty goal got started with the lofting of balloons. Nikken Sekkei's team floated 50 from the site to ascertain wind patterns at increasing altitudes.
The company had previously designed many tall towers, including, under the direction of Waseda University's Tachu Naito, the 333-meter-tall Tokyo Tower, which was completed in 1958.
Tokyo Tower, the current transmitter of TV signals for the city, which will still be in use until 2013, was built to withstand a wind speed of 90 meters per second at its top. However, Tokyo Sky Tree, which will begin transmitting six channels in January next year, and another one in April, is nearly twice as high as Tokyo Tower — a crucial factor considering that wind speed is generally higher the greater the altitude. "Understanding patterns over the site was essential in preparing a wind-resistant design for this tower," says Kamei.
Numerous computer analyses were performed. Kamei says that though the structure appears simple, three-dimensional simulations utilized approximately 700,000 data points.
To mitigate winds at the top, a "tuned-mass damping" system has been installed. In practice, this translates into two massive ballast weights, one 25 tons and the other 40 tons, that were supplied by Mitsubishi Heavy Industries and suspended near the top with large springs and dampers. Similar to the concept employed with the concrete core and outer truss frame below, these two counterweights work to offset lateral movement.
"They are the largest springs we've fabricated," Yoshiki Watanabe, president of Osaka-based Tokaibane Manufacturing, told weekly tabloid Josei Seven (May 10-17). The colossal coils weigh one ton each and measure 8 cm in diameter.
Back on terra firma, the tower's foundation is "anchored to the ground like a giant tree," as the Nikken Sekkei website explains. Sets of cylindrical steel and thin-walled concrete piles extend up to 50 meters beneath the surface in an arrangement similar to the spikes on golf or athletics shoes.
All told, the criteria for ensuring safety from a natural disaster is impressive. The tower has been built to withstand a magnitude 6.9 earthquake sourced from a presently unknown fault directly beneath it; while Tokaibane's Watanabe says his company was also required to take into account Typhoon Muroto, which struck the Kansai region of western Japan on Sept. 21, 1934, with powerful winds that killed more than 2,700 people.
Videos on YouTube shot from 2:46 p.m. on March 11 last year — the moment the Great East Japan Earthquake began — show the then 625-meter tower swaying, with one of its tall cranes used in the construction process swinging violently close to its topmost point.
Shigeaki Tabuchi, site director for Obayashi, has since been quoted in the media saying that the top of the tower was displaced by between 4 and 6 meters during the March 11 quake. Yet the only substantial problem for the project was a two-month hiatus in the construction schedule due to delayed material deliveries brought about by disaster-related supply-chain problems.
Of the Sky Tree's appearance, considerable attention was paid to creating a "lightness of volume" to reduce the amount of steel required and any feeling of oppressiveness to the eyes of local residents. Consequently, the high-strength steel pipes welded vertically, horizontally and diagonally that compose the visible external truss network combine engineered strength with a pleasing, unweighty wickerwork aesthetic.
As well, Kamei points out that a structure in which a triangular cross-section at the base transitions into a circle higher up exists nowhere else in the world. Here, though, it is a feature born of necessity, due to the constraints of the site.
Commenting on this, Kamei observes, "Sometimes, unique designs are not rational from a structural viewpoint, but a tower exceeding 600 meters in height has to be structurally rational if we take into account the budget" — which as of the latest data available, for 2010, was ¥59.6 billion.
Tokyo Tower presently is the symbolic king of the skyline, with a design and appearance from near or far that has provided inspiration for books and films.
However, Kamei envisions that future generations will accord the Sky Tree an appreciation not unlike that for the pyramids of Giza, the towers of San Gimignano in Italy or the Gateway Arch in St. Louis, Missouri. "I think people tend to preserve monuments which have original shapes and characteristics," the architect says.
The Sky Tree has, though, already proved to be a facilitator of dreams. In February, the construction company Obayashi, motivated by its work on the project, unveiled plans to build a "space elevator" by the year 2050. Such a contraption, first conceived in the 19th century, would send an elevator compartment traveling along the outside of a heavy-duty carbon-fiber cable, much like a beanstalk, extended from Earth into space.
Yoshio Aoki, a professor in the department of precision machinery engineering at Tokyo-based Nihon University, who is the director of the Japan Space Elevator Association, sees the Sky Tree as a test case for propelling a manned elevator to the stars.
"For elevators in high towers, like the Sky Tree, the hoist ways are not wrapped by walls," says the professor of what would be the ultimate pie, or castle, in the sky. "This is the same as a space elevator."
Aoki says that if the elevators in the Sky Tree — which are in glass-enclosed shafts on the outside of the inner core — can endure wind, rain and other elements, and can function continuously, then the concept can be applied to a space elevator.
"That will be the proof we need," he says.
So, for the "Son of Sky Tree," it seems, the sky really may not be the limit.
Brett Bull is editor in chief of The Tokyo Reporter, at www.tokyoreporter.com.