What allows pagodas to withstand seismic waves in Japan?

Reuters

A reason to be jittery

IN A state swept past typhoons and shaken past earthquakes, how have Nihon'south tallest and seemingly flimsiest one-time buildings—500 or and then wooden pagodas—remained standing for centuries? Records show that simply two have collapsed during the past 1,400 years. Those that take disappeared—and many take—were destroyed by fire as a issue of lightning or civil state of war. The disastrous Hanshin earthquake in 1995 killed 6,400 people, toppled elevated highways, flattened office blocks and devastated the port surface area of Kobe. All the same information technology left the magnificent v-storey pagoda at the Toji Temple in nearby Kyoto unscathed, though information technology levelled a number of lower buildings in the neighbourhood.

Japanese scholars have been mystified for ages about why these alpine, slender buildings are so stable. In earthquake-jittery Japan, information technology was only 30 years agone that the building industry felt confident plenty to erect office blocks of steel and reinforced concrete that had more than a dozen floors. With its special stupor absorbers to dampen sudden sideways movements from an earthquake, the 36-storey Kasumigaseki building in central Tokyo—Nippon's beginning skyscraper—was considered a masterpiece of seismic engineering when built in 1968.

Notwithstanding in 826, with simply pegs and wedges to keep his wooden structure upright, the master builder Kobodaishi had no hesitation in sending his purple Toji pagoda soaring 55 metres (180ft) into the heaven—almost one-half every bit loftier as the Kasumigaseki skyscraper built some eleven centuries afterward. Though it burned downward four times after being struck past lightning, the latest version of Kobodaishi's classic construction has stood its ground since 1644. The slightly smaller Horyuji pagoda in nearby Nara, originally built in 607, is considered the oldest multi-storey wooden structure in the earth. Conspicuously, Japanese carpenters of the day knew a few tricks nigh allowing a building to sway and settle itself rather than fight nature's forces and lose disastrously. Merely what sort of tricks?

No stairs, no lifts

The multi-storey pagoda came to Nihon from China in the sixth century, with the introduction of Buddhism. The Chinese built their pagodas mainly in stone, with inner staircases, and used them equally much every bit scout-towers as for worship. In Japan, all the same, the architecture was freely adjusted to meet the local weather.

The Japanese stuck with forest—and they saw no reason to clutter the design with an inner staircase. The upper floors of a Japanese pagoda serve no practical purpose. Often, in fact, there are not even stairs to them. According to Atsushi Ueda, a professor of architecture at Kyoto Seika College, "the Japanese pagoda has evolved from an observation belfry to a tower that is itself observed—a kind of objet d'fine art."

Earthquakes bated, the biggest deviation between the Japanese archipelago and the mainland to the north-west is the amount of rainfall. Each summertime, the Japanese islands get battered by a couple of dozen typhoons that swirl up from the Philippines. Having to cope with more twice the annual rainfall of China, Japanese builders take long learned to extend the eaves of their buildings much farther out from the walls. This prevents rainwater from gushing down the walls and into the foundations, softening the soil and causing the building somewhen to subside or fifty-fifty collapse. Pagodas in China and Korea have nada like the overhang that is institute on pagodas in Nihon.

Using a novel arrangement of staggered, cantilevered beams to prop upward the eaves, the roof of a Japanese temple building can be made to overhang the sides of the structure by 50% or more of the building'southward overall width (see chart). A bonus is that the successive cantilevering causes the gradient of the roof to become progressively shallower from the heart out, creating the gentle "catenary" roof line that is unique to traditional buildings in Nihon. Many Japanese notice this a particularly pleasing shape, reminding them of Mount Fuji's sloping sides or the drooping branches of a matsu pino tree.

Glace like a snake

And then far, then sensible. But why the resilience? Is the answer that, like a tall pino tree, the Japanese pagoda—with its massive trunk-like central pillar known as a shinbashira—simply flexes and sways when riding out a typhoon or an earthquake? For centuries, many idea and then. Only the respond is not so simple.

A number of things ensure that a pagoda works cypher like a pino tree. The about startling one is that the trunk-similar shinbashira carries no load at all. In some pagoda designs, it does not even remainder on the ground, merely is suspended from the top of the pagoda—hanging loosely in a well down through the middle of the edifice. The weight of the building is supported entirely by 12 stout outer columns, which form the square perimeter of the building, plus four inner columns at the corners of a smaller, inner square.

Another bizarre feature of the pagoda is that, because the building tapers, with each successive square floor-plan beingness smaller than the one beneath, none of the vertical pillars that carry the weight of the building is connected to its corresponding pillar above. In other words, a five-storey pagoda contains not even one pillar that travels correct up through the building to carry the structural loads from the pinnacle to the bottom. Such a design would non be permitted under Japan'south electric current edifice codes. Wooden buildings with ii or iii storeys today must have through-pillars connecting the roof firmly to the foundations.

A third peculiarity about Japanese pagodas is that the wide eaves that overhang their individual storeys are deliberately loaded downwards with heavy earthenware tiles. If they were branches of a tree weighed down with snow, they would be torn from the torso in the offset breeze. It would be an piece of cake matter to make the eaves much lighter—by using, say, wooden shingles instead of tiles. And then the weight as well as the size of the eaves must exist a clue to the pagoda'south ability to survive.

Another feature that makes a pagoda unlike a tree is that, curiouser and curiouser, its individual storeys are non actually attached to one another. They are simply stacked one on top of some other similar a pile of hats. What joints there are between the floors are loosely fitting wooden brackets that permit each storey to slither around.

1 farther oddity needs resolving before one can pinpoint the source of the pagoda's stability. Put simply, if the shinbashira plays no structural role, what on earth does it actually do? It is by far the largest piece of timber in the edifice. And being fabricated from the straightest of the direct-grained hinoki tree (Japanese cypress), information technology is a formidably expensive slice of wood. It cannot be simply to support the "finial"—the nine-ring ornamentation and sacred gem that adorns the top of a pagoda. There are far easier ways of attaching a decorative spire to the top of a building. More telling notwithstanding, the shinbashira is strictly a Japanese invention. Information technology is non establish in pagodas elsewhere. And so now we take the last—and most significant—inkling to the mystery of the Japanese pagoda.

Pendulum without a swing

The best way to empathise the shinbashira'southward office is to watch a video fabricated by Shuzo Ishida, a structural engineer at Kyoto Plant of Engineering. Mr Ishida, known to his students as "Professor Pagoda" considering of his constant passion to sympathize the edifice'due south unique dynamics, has built a series of models and tested them on a milk shake-table in his laboratory.

Some of Mr Ishida's model pagodas have a simulated shinbashira attached to the ground, as was common in pagodas congenital during the 6th to eighth centuries. Others simulate afterward designs with the shinbashira resting on a beam on the second floor or suspended from the fifth. Compared with a model with no shinbashira at all, Mr Ishida finds that the one with a central column anchored to the footing survives longest, and is at least twice every bit potent every bit any other shinbashira arrangement.

What the early craftsmen had found by trial and mistake was that, given a hefty sideways shove, a pagoda'south loose stack of individual floors could be made to slither sideways to and fro independent of one another. Viewed from the side, the pagoda appeared to being doing a ophidian dance—with each sequent floor moving in the opposite direction to the ones immediately in a higher place and below. Simply if a big fatty shinbashira ran up through a hole in the center of the building like a very loosely tightened bolt, each storey would then exist constrained from swinging too far in any direction by banging internally against this central fixture. Better notwithstanding, each time a storey collided internally with the shinbashira, information technology would dump some of its energy into the massive key pillar, which could and then disperse it safely into the ground.

In short, the shinbashira was acting like an enormous stationary pendulum, which the puny shoves from the separately oscillating floors were futilely trying to make swing. Though they had none of the mathematics, the aboriginal craftsmen seemed to accept an innate grasp of the principles behind what today is known as "tuned mass damping". This is the mechanism which allows the Kasumigaseki building to ride out a vehement earthquake.

And what of the extra-wide eaves with their heavy tiles? Recall of them as a tightrope walker's balancing pole. Because of inertial effects, the bigger the mass at each finish of the pole, the easier information technology is for the tightrope walker to maintain his residue. The aforementioned holds true for a pagoda. "With the eaves extending out on all sides like balancing poles," says Mr Ueda, "the building responds to even the most powerful jolt of an earthquake with a svelte swaying, never an sharp shaking." Here again, Japanese master builders of a yard years ago anticipated concepts of modern structural technology: stiffness, moment of inertia, and radius of gyration.

One last riddle

And then the underground of the Japanese pagoda's enduring strength and stability is out. It is in outcome the sum of three mutually reinforcing factors: the inertia of its actress-wide eaves, the freedom of the loosely-stacked storeys to slither to and fro independent of 1 another, and, higher up all, the energy-absorbing capacity of the ingenious shinbashira. Together, the whole is a quite extraordinary feat of structural engineering, using poise and residual in place of fauna strength.

And then ingenious is it that i cannot but wonder: did the Japanese craftsmen of xiv centuries ago stumble on the concept of the shinbashira by accident, or past inspiration? All one tin say is that the idea of an upright wooden pillar with one terminate cached in the ground to tap the spirits of the inner world has been a religious symbol in Japan since the earliest of times. Contempo excavations of Jomon sites dating dorsum to 10,000BC have unearthed evidence of wooden posts existence used for ceremonial purposes. Ane modern view is that, like the early Christians in Europe, the early Buddhists in Japan sensibly incorporated much of the native organized religion that went before.

If that were the case, the early pagodas may have incorporated the Indian symbolism of the stupa with the early Japanese symbolism of the wooden post. The idea that the shinbashira could be as much a religious object equally a dynamic balancing device for dampening the subversive forces of earthquakes and typhoons is attractively reassuring. God is in the details, after all.

This article appeared in the Christmas Specials department of the print edition under the headline "Why pagodas don't fall downwardly"

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Source: https://www.economist.com/christmas-specials/1997/12/18/why-pagodas-dont-fall-down

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