A tripod, a shape-changing section, and a 1,300-year-old anti-earthquake trick
How Tokyo Skytree was built
Skytree had to be very tall, fit on a cramped riverside plot, and survive the earthquakes that periodically flatten this part of the world. The solutions to those three problems are visible in the finished tower if you know what to look for.
Book your Skytree entry ↗Why it exists at all
Tokyo Tower had broadcast Tokyo's television signal since 1958, but as high-rise buildings multiplied through the 1990s and 2000s its 333 metres were no longer enough to clear them, and the switch to digital broadcasting made the problem urgent. Skytree was built to put the transmitters high enough to reach the whole metropolitan area again. The observation decks are, strictly speaking, a by-product.
The tripod, and why
The available site was a cramped plot beside the Sumida, hemmed in by railway lines. A conventional square footing would not fit. The solution was a three-legged base, roughly 68 metres to a side, which spreads the load across a smaller footprint. The legs sit on continuous underground walls with projections shaped like the roots of a tree, gripping the ground rather than merely resting on it.
Triangle to circle
The tripod means the tower's cross-section at ground level is a triangle. But a circular section is better for a tall tower — it presents the same profile to wind from any direction. So the section changes continuously as it rises, from triangle at the base through progressively rounded forms to a true circle at around 320 metres. Nothing is stacked; the steel is twisted through the transition.
Sori and mukuri
That transition produces a side effect the designers deliberately exploited. Because the section is rotating and shrinking at the same time, the tower's outline is not straight: seen from some angles it curves inward like sori, the concave sweep of a Japanese sword blade, and from others it swells slightly outward like mukuri, the convex profile of a temple column or an eave. Both are traditional forms in Japanese architecture, and which one you see depends on where you stand.
The pagoda in the core
The most interesting piece of engineering is invisible. A reinforced-concrete shaft runs up the centre of the tower, containing the stairs and lift shafts. It is tied to the outer steel frame only up to about 125 metres; above that it hangs free, connected through oil dampers. In an earthquake the core and the frame therefore swing at different periods and partly cancel one another, cutting the tower's movement substantially.
Where that idea came from
From the five-storey wooden pagoda, which has an unusual survival record in Japanese earthquakes. A pagoda has a shinbashira, a central pillar that runs up through the structure without being rigidly fixed to the floors around it, so the storeys move independently and the whole thing writhes rather than snapping. Skytree's engineers took the principle directly and executed it in concrete and steel at forty times the scale.
It was tested during construction
The Tohoku earthquake of March 2011 struck while the tower was essentially complete but not yet open. The structure came through it without damage, which is about as convincing a proof of the design as anyone could ask for, and delayed the opening only by a matter of months.
634 metres, deliberately
The final height was chosen for the pun — six-three-four reads as mu-sa-shi, the old province name for the Tokyo region. It also comfortably made Skytree the tallest tower in the world, ahead of the Canton Tower in Guangzhou, and second only to the Burj Khalifa among all structures.
The timeline
Construction began in July 2008, the tower reached its full 634 metres in March 2011, and it opened to the public on 22 May 2012. The colour of the steel, an off-white with a faint blue cast, was specified for the project and is called Skytree White; it is based on aijiro, a traditional Japanese pale indigo.