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Aug 13
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architecture, buddhism, japan
• economist.com
why japanese pagodas don't fall down
resilient instabilities


In having to cope with more than twice the annual rainfall of China, Japanese carpenters have long learned to extend the eaves of their buildings much farther out from the walls. This prevents rainwater from gushing down the side of the building into the foundations, causing the ground to soften and the building to subside or even collapse. The eaves of a Japanese pagoda extend the building's width by 70% or so. In China, they rarely overhang by more than 20%.
Looked at from a distance, Japanese pagodas always remind your correspondent of a matsu pine tree, with its branches drooping gently under the weight of winter snow. Indeed, for centuries, people believed the pagoda got its resilience from its central pillar, known as a shinbashira. When struck by an earthquake or typhoon, this was thought to flex and sway like the trunk of a pine tree in a storm.
But a pagoda works nothing like a pine tree. Indeed, far from carrying the building's main loads, as the trunk of a tree does, the shinbashira--made from precious straight-grained Japanese cypress--is not even rooted in the soil. In some instances, it dangles freely above the ground; in others, it merely rests lightly on the foundations or second floor.
Even more strangely, the individual floors of a pagoda are not rigidly attached to those immediately above and below. They are simply stacked on top of one another like a pile of hats. The only force holding them together is gravity. Meanwhile, their loosely fitted joints allow each storey to slide horizontally independently of the others.
So, why don't they topple over at the first tremor? For two reasons. First, as the structure begins to sway, the heavy-tiled roof covering the extended eaves of each storey acts like the long pole with weights on the ends that a tightrope walker uses to steady himself. In both, the large "radius of gyration" means the shaking has a lot of inbuilt inertia to overcome.
Second, as the loosely stacked storeys slide to and fro--with each consecutive floor moving in the opposite direction to the one above and below--they collide internally with the trunk-like shinbashira dangling through the central well of the building. With each collision, they dump more of their kinetic energy into the massive column--trying vainly to make it swing like a pendulum. . . .