Japan

Mysteries of Japanese pagodas unlocked
By Edwin Karmiol

TOKYO - Despite being built without nails or reinforcing metal parts, some 400 wooden multi-story Buddhist pagodas across Japan have, throughout the centuries, withstood destructive earthquakes and powerful typhoons. Modern builders are now using computers in an effort to apply the ancient building techniques to today's highrises.

In the aftermath of the 1995 Great Hanshin Earthquake in Hyogo prefecture, less than 600 kilometers southwest of Tokyo, all 15 three- and five-story pagodas in the devastated areas remained undamaged despite being buffeted. While more recently built structures were falling around them, these magnificent old pagodas remained intact.

The sturdiness of these structures is due to their flexibility, achieved through their assembly method. In fact, during tremors and powerful winds, the tiers of the pagodas move independently as if performing an ancient snake dance.

Engineers at Takenaka Construction Co have, through computer simulation, confirmed how the 7th century, five-story pagoda at Horyuji temple in Nara prefecture could resist an earthquake as strong as the one that destroyed parts of the city of Kobe in 1995.

Toshiharu Hisatoku, until recently an executive architect with Takenaka Corp and leader of the quake-simulation project, explained the mechanism that allowed the 32.55-meter-tall, 305-tonne pagoda to survive for the past 1,300 years.

The existing pagoda, along with other Buddhist temples at the site, was founded by the emperor's son, Prince Regent Shotoku (574-622), but was reconstructed around 711 after the original one burned down. Fire is the only natural enemy of these edifices built of strong Japanese cypress wood.

All five tiers of that pagoda, and in all pagodas, are structurally independent from one another. A central pillar, or shimbashira, standing on a base stone extends up to the spire but does not support the framework, roof, or any other parts of the structure.

It is the inner and outer posts around the central pillar that are instrumental in supporting the weight of each story independently. Consequently, the fifth-floor roof is supported by posts resting on the base rafters of the fourth floor while the latter posts sit on the third floor's base rafters.

This, said Hisatoku, "imparts a great flexibility to the structure as the overhanging eaves function like a mobile, creating a counterbalanced lever".

In case of an earthquake or typhoon, each of the pagoda's five tiers vibrates and moves alternately in the opposite direction - so that when the first story of the pagoda sways to the left, the second floor swings to the right, and the third story rocks in the opposite direction, and same with the other floors.

This snake-like movement re-establishes the balance of the structure in a natural way. Moreover, the central column serves as a stopper for the base rafters battering against it whenever they are swaying too violently. This movement prevents the pagoda from staggering too far out of line and collapsing.

Yoneko Oyama, a resident of the city of Nara, says: "It gives me a sense of security; it is well designed." Oyama, who teaches volunteer tour guides in Nara, adds that most Japanese people are unaware of the anti-earthquake properties of the pagodas.

"My students are amazed when I familiarize them with such details. I feel some responsibility to teach them about the meaning of pagodas. I relay this information to them in the hope that they will pass it on to the younger generation," she said.

In computer simulated experiments, Hisatoku's team entered seismic-wave data to trigger a major "quake" measuring 7.2 on the Richter scale directly beneath the Horyuji temple pagoda. It also programmed the computer to simulate horizontal and vertical tremors at a rate of 0.9 times per second and 2.5 times per second, respectively. Despite the magnitude of the "quake", the pagoda's top tier oscillated horizontally only 38 centimeters.

It is believed that structures topple when they move horizontally by one-50th to one-30th of their height. It is also believed that the looseness and mobility of the joints in wooden structures help absorb the earthquake's violent energy, keeping it from toppling.

These results convinced Hisotaku that "even wooden houses could resist earthquakes if they are built with the same elastic characteristics intrinsic to the pagodas".

Reinforcing the flexibility theory, are observations by Tsunekazu Nishioka, a temple carpenter who has supervised the restoration of a pagoda. "The building looks very sturdy and solid, but if you give it a good shove with your hand while the pagoda is still under construction, the whole structure sways back and forth. If it doesn't, the building is no good," he said.

Taking their inspiration from the earthquake-resistant features of pagodas, Japan's leading construction companies have borrowed and improved these techniques in modern construction. Highrise structures are now being gradually outfitted with the most advanced anti-earthquake technologies.

Takenaka Corp's 161-meter-high Applause Tower in Osaka, 550km southwest of Tokyo, which was completed in 1992, has a heliport on top of the tower that has incorporated a 480-tonne active mass damper (AMD) system. The computer-activated AMD can absorb the swaying of a building in case of strong winds and earthquakes. Whenever the sensors, located on different floors, detect tremors, the data are relayed to a computer system that triggers the actuator to shift its weight in the opposite direction of the seismic motions.

Takenaka says this helps reduce the earthquake and strong wind vibrations by 50 percent. The AMD concept is based on the pagoda's "snake dance", says Hisatoku.

In Japan, engineers have drawn practical ideas from the pagoda's resistance to earthquakes, even if they are still not sure of having probed all its secrets. "It is doubtful that the carpenters who build the pagodas knew themselves how to make them quakeproof. It is maybe sheer luck," said Hisatoku.

(Inter Press Service)

 
Aug 17, 2002



 

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