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Some of the world's most famous structures are also some of the tallest. New York City's Empire State Building measures 1,250 ft. (381m). The Petronas Towers in Kuala Lumpur are 1,483 ft. (452m). Taiwan's Taipei 101 reaches 1,667 ft. (508m). The Burj Dubai, however, is on target to reign supreme: Scheduled for completion in 2009, it will stretch an incredible 2,684 ft. (818m) into the sky.
But did you know Japan has ancient “skyscrapers”? The five-story pagoda at Horyuji Temple, a UNESCO World Heritage Site in Nara, is not only the world's oldest surviving wooden building (built approximately 1,300 years ago), it might very well be considered the world's first skyscraper: It rises approximately 30m (98.5 ft.) from a base structure.
In this fabled land of earthquakes, why is the Horyuji pagoda still standing? Why, indeed, have 22 five-story pagodas around Japan remained intact? Research is under way to reveal what makes their design so resilient.
Traditional theories suggest that the thick central column, or shin-bashira, might stop vibrations in their tracks or that heavy rings (so-rin) at the top of the central column act to minimize vibration throughout the structure. However, “No one can fully understand why a pagoda's five-story structure improves resistance to earthquakes,” says Dr. Naohito Kawai, chief research engineer at the Department of Structural Engineering at the Building Research Institute in Tsukuba, Ibaraki, and leader of the “Shake Five-Story Pagoda Committee.”
Using the “Shaking Table” earthquake simulator at the National Research Institute, Dr. Kawai's team conducted experiments in 2004 and 2006 using a 1/5 scale model of a five-story pagoda. The experiments seemed to prove one critical fact: A pagoda has a long-wave period. The Horyuji Temple pagoda, for example, has a long natural period of 1 second, which helps protect it from the ravages of earthquakes, even the dreaded killer pulse. One of the reasons for this is its unique five-tier structure. During quakes, each of the five tiers sways in opposite directions, making the wave periods shorter. You might think that these shorter periods would coincide with those of a seismic wave, but in reality they are easily contained. In fact, the structure does not easily succumb to resonation, even during shaking produced by the killer pulse.
Basic Structure of a Five-Story Pagoda
A central column (shin-bashira) supports the rings (so-rin) on top. The roof of the fifth floor touches the central column, which is free from structural frames, and is in some cases disconnected from the base structure. The tower's weight is supported by side pillars located at each floor level. These pillars are arranged to support the floors on different axes.
“It's important to understand that a wooden building can actually have anti-earthquake properties,” Dr. Kawai says, pointing out that the organic structure of wood is one key to longevity. The fibers “give,” expanding and contracting in response to stresses and strains, acting as natural shock absorbers when the building is rocked by an earthquake. In other words, timber is limber.
A five-story pagoda is a formidable opponent against seismic waves. Even though Dr. Kawai maintains they weren't designed with earthquakes in mind, it's clear they are testament to Japanese craftsmanship acting in harmony with its surroundings. In fact, the design of these beautiful and long-lived structures — the use of a strong central column, for example — has influenced modern architecture, particularly that of the high-rise buildings that characterize modern Japan.
