YOUR correspondent is indebted to readers for their interesting comments about last week's column on timber-framed buildings. He is especially grateful to Anjin-san , whose observations about Japanese pagodas reminded him of a day spent a dozen years ago with Shuzo Ishida, a structural engineer at Kyoto Institute of Technology. Mr Ishida, known to his students as “Professor Pagoda”, has a passion for the building's unique dynamics.

What has mystified scholars over the ages is how these tall, wooden buildings cope so well with the earthquakes and typhoons that plague Japan. Many have been struck by lightning and burned to the ground. Others have been torched by marauding warlords. Fire was a perennial hazard in Japan when wood and paper buildings were the norm. But, remarkably, only two of the country's hundreds of wooden pagodas have collapsed over the past 1,400 years as a result of violent shaking.

Reuters

The disastrous Hanshin earthquake of 1995 killed more than 6,400 people, toppled elevated highways, flattened office blocks and devastated the port city of Kobe. Yet the magnitude 6.9 shock left the magnificent five-storey pagoda at the Toji temple in nearby Kyoto unscathed, even though it levelled a number of lower structures in the neighbourhood.

You can see the Toji pagoda soaring 55 metres (180 feet) into the sky from the train as it pulls into Kyoto station. Though burned down four times since it was first erected in 826 by the master builder Kobodaishi, the current building has stood its ground since 1644. It was the tallest structure in Japan until the 36-storey Kasumigaseki Building was erected in 1968. The slightly smaller Horyuji pagoda in Nara was built in 607 and is thought to be the oldest multi-storey wooden structure in the world. Today, it is on UNESCO's world-heritage list of cultural treasures.

What is it about Japanese pagodas that makes them so different from their ancestors in India and later China? All were religious monuments of one sort or another. In India, mounds of stone called “stupas” were used to house religious relics and writings. As Buddhism spread to China, the stone-built structures became taller and more elaborate. But with the introduction of Buddhism into Japan in the sixth century, the pagoda underwent a fundamental change in design.

For a start, the Japanese version is built entirely from wood rather than stone. Curiously, it has no internal staircase. Unlike its equivalents elsewhere in Asia, which were used as lookout posts as well as for storing religious objects, the Japanese pagoda serves no practical purpose. According to Atsushi Ueda, an architect, “the Japanese pagoda evolved from an observation tower to a tower that was itself observed—a kind of object d'art.”

To understand why Japanese pagodas are so different, consider first the climate. The Japanese archipelago is on the northern fringe of the monsoon belt. It also gets drenched each year by a dozen or more typhoons that swirl up from the Philippines.

In having to cope with more than twice the annual rainfall of China, Japanese carpenters have long learned to extend the eaves of their buildings much farther out from the walls. This prevents rainwater from gushing down the side of the building into the foundations, causing the ground to soften and the building to subside or even collapse. The eaves of a Japanese pagoda extend the building's width by 70% or so. In China, they rarely overhang by more than 20%.

Looked at from a distance, Japanese pagodas always remind your correspondent of a matsu pine tree, with its branches drooping gently under the weight of winter snow. Indeed, for centuries, people believed the pagoda got its resilience from its central pillar, known as a shinbashira. When struck by an earthquake or typhoon, this was thought to flex and sway like the trunk of a pine tree in a storm.

But a pagoda works nothing like a pine tree. Indeed, far from carrying the building's main loads, as the trunk of a tree does, the shinbashira—made from precious straight-grained Japanese cypress—is not even rooted in the soil. In some instances, it dangles freely above the ground; in others, it merely rests lightly on the foundations or second floor.

Even more strangely, the individual floors of a pagoda are not rigidly attached to those immediately above and below. They are simply stacked on top of one another like a pile of hats. The only force holding them together is gravity. Meanwhile, their loosely fitted joints allow each storey to slide horizontally independently of the others.

So, why don't they topple over at the first tremor? For two reasons. First, as the structure begins to sway, the heavy-tiled roof covering the extended eaves of each storey acts like the long pole with weights on the ends that a tightrope walker uses to steady himself. In both, the large “radius of gyration” means the shaking has a lot of inbuilt inertia to overcome.

Second, as the loosely stacked storeys slide to and fro—with each consecutive floor moving in the opposite direction to the one above and below—they collide internally with the trunk-like shinbashira dangling through the central well of the building. With each collision, they dump more of their kinetic energy into the massive column—trying vainly to make it swing like a pendulum.

While at Kyoto Institute of Technology a dozen years ago, Mr Ishida showed your correspondent a series of models he had made of five-storey pagodas—some with central columns that rested on the ground, others that were anchored at the second-floor or hung freely from the top the structure. When given a hefty shove on a shake-table, all of them proved more stable than a test model that had no central column. But the one that damped the swaying best had its shinbashira resting firmly on the ground.

In practice, the mighty shinbashira either conveys the surplus energy of the swaying pagoda to the ground by dragging its base over the foundations—or, if dangling freely, begrudgingly swings a little. In essence, then, the pagoda is a tuned mass-damping system.

Like all great buildings, a Japanese pagoda is as much a machine as a structure. Engineers distinguish carefully between the two. Great pains are usually taken to ensure that structures, although flexible, are incapable of swivelling or sliding at their joints as machines do. Otherwise, they may collapse in a heap.

What the pagoda builders realised was that they could use controlled motion at the joints of a building to help it dissipate sudden stresses imposed on its various members. Thanks to their pegged joints, the timber-framed buildings your correspondent described last week cope with subsidence by “giving” a little at the joints—albeit with nothing like the freedom of movement a pagoda has. How clever of those Japanese craftsmen to figure it all out 14 centuries ago.