This was not a result of divine protection. Rather, studies on the behaviour of pagodas in earthquakes have shown that their carpenters intuitively developed a way to prevent their work being flattened.
Now a team of structural engineers at Arup is employing similar techniques on the stunning steel structure of a retail and office building designed by architectural guru Renzo Piano for French fashion house Hermes in downtown Tokyo.
"There is a direct parallel between the way pagodas are constructed and what we've designed here," says Bob Lang, the structural engineer who is heading Arup's team working on the building.
The Hermes project, which was due to open for business yesterday, is squeezed onto a tight corner site on Tokyo's equivalent to Oxford Street in London. To use the plot to its full and to maximise street frontage, the 12 m wide, 13-storey high, wafer-thin building stretches for more than 50 m along its main elevation. The narrow building is clad in a unique transparent skin constructed from 450 mm square glass blocks, which appear to glow from within.
The challenge for the Arup team was to ensure that as little as possible of the shock of an earthquake would be absorbed by the structure of this slender building.
In describing the design, however, it is necessary to understand how a traditional Japanese wooden pagoda behaves in an earthquake.
Each storey of a pagoda is covered by a roof supported on a series of wooden columns. However, because the building tapers upward, none of them are connected to the corresponding column either above or below. This means that each storey is simply stacked on the one below and can move independently of it.
A thick central pillar, or shinbashira, runs through the centre of the building from the ground floor to the finial crowning the uppermost roof. Under normal conditions, this carries surprisingly little load.
What makes it possible for the pagoda to survive an earthquake is that the joints in the structure are loose, allowing it to deform without causing catastrophic collapse. In an earthquake, the lowest storey columns on the side of the impact lift from the ground, leaving that floor supported by the shinbashira and one set of legs.
However, each storey will move in the opposite direction to the storey above and below, so that the whole structure waves like a willow branch, or, as the Japanese say, a "snake dance".
This is where the shinbashira comes into its own: it acts like a long bolt running through the five storeys to stop them moving too far.
Lang and a specialist Arup team located in London, Tokyo and San Francisco, have taken this concept of allowing a building to flex in an earthquake and applied it to the slender Hermes building.
"In broad terms, the force a structure will feel from an earthquake is proportional to its stiffness – the stiffer the structure the greater the force," says Lang. He uses car suspension as an analogy. "A car with a hard suspension system will jolt when it goes over a little bump, but a car with a soft suspension system will pass over without the bump being felt by its occupants."
However, the engineers could not make the structure too flexible – Japanese building regulations limit the amount a building can move to prevent adjacent buildings colliding in the event of an earthquake.
So, the final design combines a flexible structure with specially designed damping to reduce further the force of the earthquake on the building's structure.
Vertical support in the building is provided by three rows of columns spaced at 3.6 m centres along its length. Damping is incorporated in the rear line of columns in the form of sheets of visco-elastic material clamped between steel plates, which Arup designed specifically for this scheme.
These visco-elastic dampers allow the column to lift vertically, reducing the building's stiffness – and consequently the earthquake's force on the building – in the same way as the feet of the pagodas.
In the centre of the building is a second row of columns, from which structural arms reach out and span the width of the building to support the floor plates. A further row of articulated steel columns at the front runs vertically up the building to tie these arms together.
The structure is designed to pivot around the building's centre row of columns, which effectively become the building's shinbashira.
As the earthquake's force hits the building, the building flexes and the dampers come into play. These columns lift up slightly, stretching the dampers and so absorbing some of the energy. This also has the effect of reducing the pulling force on the foundation.
As the columns at the rear lift up, the building's structure will pivot around the centre row of columns. This in turn will compress and stretch the articulated row of columns at the front.
This Tokyo building provides a world first try-out of such a system. If the design theory works, it will prove to be a significant step forward in minimising damage caused by earthquakes.
