A recent picture of the Sky Tree under construction, at a height of 398 meters.

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Testing the earthquake resistance of a five-story pagoda.
(C)NPO Corporation Forum for Wood Architecture / Building Research Institute

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Construction work on the Tokyo Sky Tree in the Sumida City, Tokyo of the Japanese capital is moving forward rapidly. The Sky Tree is a digital terrestrial broadcast tower being built to reduce interference to signals from the high-rise buildings. The structure is already 398 meters high; when it opens to the public in spring 2012 the Sky Tree will be the tallest free-standing broadcast tower in the world, at a height of 634 meters. (One Japanese reading of the numbers 6-3-4 is similar to the pronunciation of Musashi Province, the old name for the area in which the tower is being built.) The antiseismic technologies used in the tower represent a groundbreaking fusion of traditional techniques and the very latest modern technology.

An image of the completed Sky Tree.
(C)TOBU Railway CO.,LTD and TOBU TOWER SKY TREE Co.,Ltd

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Learning from the AncientsThe tower is a two-part structure, comprising an outer steel frame and a cylindrical inner shaft made of reinforced concrete. The idea is that the two elements are independent of one another: if the two pieces were joined together, this would amplify the sway of an earthquake. Because the outer frame and the inner shaft move separately, their seismic energies cancel each other out, reducing the amount of energy brought to bear on the tower by as much as 40%.

The sway-reducing technology incorporated in the central shaft was inspired by Japan's ancient Buddhist architecture, such as five-story pagodas. As far as records show, none of these pagodas—including that at the Horyuji temple in Nara built 1,400 years ago—has ever been toppled by an earthquake; clearly something in their structure makes them resilient to seismic disturbances. Various theories have been put forward to explain this, but the independent central shaft in the structure of the pagoda is thought to play a significant role in making them resistant to earthquakes. In the Tokyo Sky Tree, the inner shaft incorporates an emergency stairway. The Sky Tree's unique seismic reduction features are testimony to Japan's position at the very front of anti-earthquake technology.

Roots of SteelEarthquakes also exert extremely strong forces on the foundations of buildings. To protect the Sky Tree against these forces, the pilings in the foundations are not straight poles but walls of steel-concrete reinforced with spike-like protuberances to increase their frictional resistance. These protuberances act like the spikes on sports shoes and help to reduce the pressure that might otherwise cause the building to collapse. The pilings are driven 50 meters into the ground and then fan out radially like the roots of a tree to bind them strongly to the surrounding earth. Just like a real tree, the Sky Tree is rooted firmly in the ground. To reinforce the structure even further, the outer frame incorporates a triangular framework using steel pipes that are twice as strong as regular pipes. The base of the tower is stabilized by giant steel bars 2.3 meters in diameter and 10cm thick.

A computer-generated image of the Sky Tree.
(C) OBAYASHI CORPORATION

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Simulation tests suggest that the Sky Tree would suffer almost no damage even in the event of a major 7.9 magnitude quake. Equipped with two observation decks and numerous restaurants and shops, the Tokyo Sky Tree is set to open in spring 2012, when it is expected to become a new landmark and symbol of the city. (July 2010)