Today, pagodas and earthquakes. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
Japan is earthquake country.
For centuries no one made multistory buildings in
Japan. Not 'til 1968 did Japanese engineers have
enough confidence to erect a 36-story,
earthquake-resistant building in Tokyo.
Yet one kind of high-rise building had been common
in Japan for over 1400 years. Scattered over Japan
are 500 pagodas that rise vertically to heights of
as much as 180 feet. Those pagodas have been
rattled by countless earthquakes, and they still stand, serene and unruffled. You know the image:
towers with multiple roofs stacked above one another. They're like Christmas trees, rising to a
decorative spire. You expect to find a star on top.
When you see how pagodas are built, you think
you've found a recipe for collapse. Each roof
carries heavy tiles. A central pole, the trunk of a
huge cedar tree, is the core on which the structure
hangs. But that pole isn't anchored in the earth.
In fact, it sometimes doesn't even reach all the
way to the ground. The weight of a pagoda is
carried entirely on supporting pillars which form a
small open porch on which the pagoda sits. It seems
to make no sense, yet pagodas stand, centuries in and
centuries out.
Now The Economist magazine goes to Shuzo
Ishida at the Kyoto Institute of Technology. Ishida
is called "Professor Pagoda," for he knows more
about those mysterious structures than anyone
living. He explains their peculiar damping
properties: When an earthquake shakes a pagoda,
those massive roofs try to slide sideways, but the
pole won't let any roof move very far. It flexes
sideways, bending to drive neighboring roofs the
opposite way. During an earthquake, the pagoda does
a kind of shimmy, or snake-dance, with each roof
damping out the motion of its neighbors.
The next time you drive past a large power
transmission line tower, notice the way a slack
length of cable often links the power line on one
side to that on the other. The purpose of that
cable is to pass wind-driven vibrations, out of
phase, from one side to the other. The vibrations
spend themselves opposing each other.
So it is with the pagoda. Buddhist philosophy tells
us to let our enemies collapse under their own
weight. These Buddhist pagodas let the devastation
of an earthquake fight itself along the length of
that cedar pole -- called a
shinbashira. It's good engineering,
worked out over centuries by trial and error and
finally verified by modern dynamic analysis. The
Economist article ends with these marvelous
words:
The idea that the shinbashira could be as much
a religious object as a dynamic balancing device
for dampening the destructive forces of earthquakes
and typhoons is attractively reassuring. God is in
the details, after all.
I'm John Lienhard, at the University of
Houston, where we're interested in the way inventive
minds work.
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