Physics Fix: Why Japanese Pagodas Never Fall Down

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Need a physics fix? You’re in luck: Following up on a fascinating column about why wooden houses withstand earthquakes, the Economist tech blog looks at why so few Japanese pagodas have ever fallen down:

What has mystified scholars over the ages is how these tall, wooden buildings cope so well with the earthquakes and typhoons that plague Japan. Many have been struck by lightning and burned to the ground. Others have been torched by marauding warlords. Fire was a perennial hazard in Japan when wood and paper buildings were the norm. But, remarkably, only two of the country’s hundreds of wooden pagodas have collapsed over the past 1,400 years as a result of violent shaking.

Despite having a central pillar that sometimes doesn’t even touch the ground and floors stacked on top of one another “like a pile of hats,” a Japanese pagoda is a resiliant creation.

…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 … How clever of those Japanese craftsmen to figure it all out 14 centuries ago.

(Thanks, Bookforum.)

Source: Economist

Image by Timothy Lloyd. Licensed under Creative Commons.

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