Secrets of the Sky Tree: Quake-proofing the world's second tallest structure
March 2, 2012
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 (634 m) the tower stands nearly twice as Japan's previous tallest frame, the 1,091-ft (333-m) Tokyo Tower transmitter. It's an audacious technological feat when one considers this is at the heart of an earthquake zone.
Nikken Sekkei, the design practice behind both buildings, effectively doubled its personal best in the construction of the Sky Tree, and in doing so employed the latest technology, not to mention the expertise of "more than 100 architects, engineers and planners", to build the capital city's striking quake-resistant giant - a super-tower its designers claim offers "better safety" than any other. It has a 9:1 height to width ratio - a narrow, almost needle-like form.
Why so tall?
The Tokyo Sky Tree, like the Tokyo Tower before it, is first and foremost a television transmitter. The Tokyo Tower was plenty tall enough in 1958, and for decades after; but with the switch to digital transmission the tower's height proved insufficient for blanket coverage to the area - hence the need for a taller tower - hence, in fact, the Tokyo Sky Tree.
The Sky Tree is a tripod at ground level standing on an equilateral triangle base with sides of 68 m (223 ft). As the tower progresses upwards its cross section very quickly morphs from triangle to circle (a shape "without precedent", according to its designers), becoming a totally cylindrical tower between half and two thirds of the way up.
It started with a balloon
Before Nikken Sekkei could put pen to paper, it had to better understand conditions at an altitude of 2,000 feet. To do that the company floated a weather balloon to gather extensive wind data - wind data used to fathom the lateral wind forces that the building would have to withstand.
Meanwhile, thousands of feet below, the company undertook a "micro-motion array observation" granting insight in the minutest detail of the make-up of the earth to a depth of 3 km (1.9 miles) underground. This level of detail allows much more accurate computer simulation of building sway in earthquake conditions.
The Sky Tree's structural design relies on extremely strong steel tubes which, at the tower's base, have a diameter of 2.3 m (7.5 ft) and a thickness of 10 cm (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.
A firm footing
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 50-m (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.
Forming a triangle between the three clusters are three longer, 35-m (114-ft) deep piled walls. Using wall-shaped piles increases the useful frictional resistance with the ground. Another cluster of piles is embedded at the center of the triangle, beneath the tower's center.
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.
Source: Nikken Sekkei
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