Earthquake-proofing one of the world’s tallest structures

Leap day 2012 saw the completion of the world’s second tallest structure, the Tokyo Sky Tree television transmitter and observation tower. At 2,080 feet the tower stands nearly twice as Japan’s previous tallest frame, the 1,091-ft Tokyo Tower transmitter. It’s an audacious technological feat when one considers this is at the heart of an earthquake zone…

The Sky Tree’s structural design relies on extremely strong steel tubes which, at the tower’s base, have a diameter of 7.5 ft and a thickness of 3.9 in. These are arranged in an array of triangular trusses which, unusually for a building, employ branch joints more common on marine structures such as oil rigs.

To control vibration, Nikken Sekkei took inspiration from what, at first glance at least, seems an unlikely source: the traditional five-story Japanese pagoda. Over the centuries, hundreds of these wooden structures have withstood earthquakes and typhoons, and Nikken Sekkei claims not a single pagoda has collapsed due to a seismic event.

This inherent strength is thought to stem from the fact that the central column (or shimbashira) does not physically support any of the pagoda’s stories but instead acts as a counterweight about which the rest of the building’s structure can vibrate. Nikken Sekkei brought the concept up to date with what it calls shimbashira seishin, or center column vibration control, with the core column and surrounding steel frame connected by a flexible oil damper.

Additional resilience is achieved through an “added mass control mechanism” (or tuned mass damper) – a damping system which, in the event of an earthquake, moves out of step with the building’s structure, to keep the center of gravity as central as possible to the tower’s base. Though steel ingots, concrete, or even the buildings mechanical plant is sometimes used to this end, in what Nikken Sekkei claims is a world’s first, the Sky Tree’s core column is the added mass.

Of course, such resilience is nothing without the proper foundation, and its the Sky Tree’s foundation that gives the buildings its name. Beneath each of the tower’s three legs is a cluster of 164-ft deep walled piles with steel-reinforced concrete nodes, which Nikken Sekkei compares to the root system of a gigantic tree, “monolithically integrated” with the ground…

The Sky Tower opens to the public in May with 360-degree views of Tokyo’s Sumida ward in the foreground to the 6,500 sq miles (17,000 sq km) of the Kanto Plain beyond.

If any of our Asian readers takes the trip to the top, send me a photo. Be careful.

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