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 Post subject: Five-story Pagodas: Why Can’t Earthquakes Knock Them Down?
 Post Posted: Sun Feb 05, 2006 5:31 pm 
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Wisdom from the Distant Past
Five-story Pagodas: Why Can’t Earthquakes Knock Them Down?


@ The Island / 05Feb2006

The five-story pagoda is a strange piece of architecture. Japan experiences many serious earthquakes but there is almost no record of a pagoda being knocked over during one. The Great Hanshin Awaji Earthquake of 1995 brought down many tall modern buildings in the Kobe area, but not one of the 13 three-story pagodas in surrounding Hyogo Prefecture was damaged. What secrets protect three and five-story pagodas from earthquakes?

Image

The first secret lies in the material used-every structural part of the five-story pagoda is made of wood. When wood is subjected to a force it may bend and warp, but it does not break easily. And when the force is removed the wood returns to its former shape. Because it is flexible, it can absorb seismic stresses.

The second secret, a structural one, complements this flexibility of wood. The timbers are fastened together, with hardly a nail at all, by inserting carved thinner and narrower ends into slots. So if the ground begins to shake, the wood surfaces in these joints twist and rub against each other. This helps prevent the seismic energy from traveling far up the tower. There are about a thousand large mortise joints in a five-story pagoda, making the entire structure practically as flexible as konnyaku (see Note 1).

The third secret has to do with the layered structure of the pagoda. If you stand a long slab of konnyaku on end it will not remain upright, but five cubed pieces placed in diminishing sizes one on top of the other will. In English we say "five-story pagoda," but the Japanese word, go-ju no to ("five-layer tower"), is more accurate because the pagoda is basically a number of box-like structures laid one on top of another, much like the traditional stacked-up boxes called jubako (see Note 2). The "boxes"are all fastened together with mortise joints. When the ground shakes, each of these box layers sways lowly and independently of the others.

The fourth secret involves a wobbling effect. Each box layer permits a certain amount of gentle swaying, but if they sway too far off center they will fall over. Long ago, a carpenter expert in the construction techniques of the time happened to observe a five-story pavilion during a major earthquake. He reported that when the bottommost box layer swayed to the left, the one above it swayed to the right, the one above that one to the left, and so on. The tower was doing a kind of snake dance! (See Figure 1.) The wobbling was not unlike that of a traditional yajirobe balancing toy that has different sized layers that sway in opposite directions from one another, then return to upright. (See Figure 2.)

But you would expect that a really strong earthquake could push one box layer off its base, bringing down the entire structure. The fifth secret-perhaps the most fascinating of them all-involves a structural component that prevents this. Imagine an experiment using a "tower" of five bowls standing upside down on a tray. Nudge the tray and they will come crashing down. But if you drill a hole through the bottom of each bowl, then insert a long chopstick through the holes and secure it vertically, the bowls become a sturdy tower and remain standing even when you shake the tray a little. If one of the bowls tries to fly off sideways, it and the other ones will be held back by the chopstick. I call this the "Bowls of Columbus" (see photos on page 25, top right), after the Egg of Columbus that stood on end because a small part of the shell at one end had been removed. The vertical chopstick holds the bowls together, something like a latch bolt fastens a door, though the bolt is horizontal. The "latch bolt" in the pagoda is a thick central pillar (shinbashira) running from bottom to top (see Figure 3). If one of the box layers tries to slide off to the side, the sturdy pillar returns it to center.

During an earthquake, the central pillar will sway a little, like an upside down pendulum, counteracting the seismic force. Pagodas may have even more architectural secrets to reveal!

All of these stabilizing factors-flexibility, mortise joints, layered box-like construction, wobbling ability, and vertical latch bolt security-are combined in a structure that resembles a willow tree in the way it sways and withstands an earthquake.

Surprisingly, this remarkable, logically built type of structure has existed in Japan for well over a thousand years. If you traveled to different parts of Asia and examined Buddhist towers, you would see that some are a bit like Japan’s five-story pagodas, but the resemblance is never very close. This indicates that the unique five-story pagoda was designed in earthquake-prone Japan, with the aid of native wisdom and techniques. Structural design concepts from the Asian continent were probably combined with the pillar construction methods used in Japanese buildings since the ancient Jomon period (for example, excavations at the Sannai Maruyama site in Aomori Prefecture revealed that six large wooden posts were used to support a building).

Structural strategies found in five-story pagodas are also seen in some tall buildings today. Older, stone buildings were made sturdy and unyielding to withstand earthquakes, like an oak tree. New ones are designed to be flexible, to sway just enough to negate seismic forces, like a willow tree-and like a five-story pagoda. Huge, layered rubber laminates are placed under foundations. A damper mechanism with interlocking frame design is used for columns, beams, walls and other structural components. Partly filled water tanks are placed on the roof, so that the water swishing around during an earthquake counteracts the seismic forces.

Five-story pagodas still stand at some old temples in Japan today, waiting to greet visitors. The pagodas have kept their beauty since ancient times, fascinating all who see them. They hold secrets representing the essence of science and technology, and open up new possibilities for modern architecture.


(Courtesy Discovery Japan)


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