The Anji Stone Bridge

Floodwater has risen and fallen beneath one stone arch more times than anyone could ever count.

The river ran dry and filled again, filled and ran dry again. Across the top of the bridge rolled heavily loaded ox carts, porters with their baskets, the bare feet of running children. The woods on both banks grew up, were cut down, and grew back again. The roofs of the village went up, grew old, came down and went up again. Even the stone railings along both sides of the bridge could not hold out against time. Every 300 to 500 years or so, people had to put in a new row of railings.

Only one thing has kept its old shape. The arch.

That stone arch has spanned the river for fourteen centuries.

It is called the Anji Bridge, a name that means carrying people safely across the river. People who live nearby usually call it after the place, the Zhaozhou Bridge. The bridge stands in northern China. It was finished around the year 605, under the Sui dynasty. The man who designed it was a stonemason named Li Chun.

To understand what Li Chun did, you have to know how arched bridges were built in his time.

Back then, when anyone spoke of a stone arch, they pictured a half circle. A half circle is very strong. The stones stacked on one another wedge each other in place, and they can stand for hundreds of years without mortar. But it has one bad habit. To cross a wide river, a half circle has to rise very high, exactly as high as half the width of the river. So the bridge is as steep as a hill. By the time a loaded ox cart reached the top, the driver and the ox were both out of breath.

Li Chun did not build a half circle.

He gave his arch only a gentle curve, like a lightly stretched rope laid across the water. The main span is about 37 meters wide, yet the deck rises only a little, an easy slope to walk. The whole bridge is more than 50 meters long.

A gentle arch is easy to walk over, but it pushes its weight out toward the two banks much harder. The heavier the bridge, the more the two banks have to carry. And this is where Li Chun did something that, at first hearing, sounds like he was ruining his own work.

He cut holes through his own bridge.

At the two shoulders of the arch, where the great curve comes down to meet the bank, he opened four holes, two on each side. Each hole was a small stone arch in its own right, carefully built, with a proper curve. Seen from a distance, the bridge looks like one big arch carrying four little arches on its shoulders.

Those four holes did two jobs at once.

The first job was to shed weight. The stone that was left out weighed more than 500 metric tons. That is more than 500,000 kilograms of stone no longer pressing down on the two riverbanks. That was about one fifth of the weight of the whole bridge. The bridge became much lighter and stayed just as strong.

The second job is where the stonemason saw far ahead. In the flood season, the river rose right up to the shoulders of the arch. Against a solid bridge, that enormous mass of water has no way to go except to slam straight into its side and stay there, pushing. Against a bridge with four holes, the water finds a way through. It slips through the four little arches and flows on. The bridge does not argue with the flood. It makes way.

But Li Chun's finest idea lies where nobody can see it.

The main arch is not one solid block of stone. It is made of 28 thin slabs of limestone, each one curved, standing side by side like ribs. If they had simply been set side by side, centuries of rain and wind would have pushed them all out of line. So Li Chun locked them together with iron cramps. Each one was shaped like a swallow's tail and set across the seam between two slabs.

Those cramps held the stones together, but only just tight enough. Tight enough that no slab could slip loose. And loose enough that the whole arch could still shift a little.

This is worth a pause. The ground beneath a bridge is never completely still. It swells and shrinks, sinks a little and rises a little, year after year, quietly, for centuries. A stiff bridge, fixed rigidly to the ground, can only crack when the earth moves. A bridge that can move along with it does not crack. It only changes its shape a little, and goes on standing.

That is the real reason the bridge is still there today. Not because it is the stiffest. Because it gives in the right places.

The ground in that region has shaken many times in fourteen centuries. The river has brought more flood seasons than anyone can remember. The bridge still spans the river in the same old place.

To be fair, the bridge has not stood all those years without anyone laying a hand on it. Later generations repaired it many times, replacing the railings and the most worn slabs. At times they even had to rebuild rows of arch stones that had fallen. In the middle of the 20th century, it went through a major restoration. Most of the arch was taken apart and laid again, and the deck was paved with new stone. And today the bridge no longer carries vehicles. It is open only to people on foot, like the very first travelers who ever stepped onto it.

In 1991, the American Society of Civil Engineers added the Anji Bridge to its list of historic landmarks of civil engineering around the world. In Europe, bridges with holes like these in the shoulders of their arches appeared only many centuries later.

The story of a bridge is always the story of people who live beside a river. Wherever there is a river, people have to work out their own answer, and each land answers in its own way.

Vietnam has kept a few beautiful answers of its own. In Hoi An there is the Japanese Covered Bridge, which people in Vietnam call Chùa Cầu. Its name in Chinese characters is Lai Viễn Kiều. It is a roofed bridge with a small shrine built onto the middle of it, so crossing the river also means passing through a place of worship. In Hue there is the Thanh Toàn bridge, built of wood under a closed tiled roof. It follows an old style called "house above, bridge below," so anyone coming home from the fields in the rain has a place to sit for a while. The same problem of crossing water, and one place answers with four holes in the shoulders of a stone arch, another with a tiled roof and a seat for a traveler caught on the road.

Fourteen centuries is a number that is hard to picture. But it shrinks back to human size for anyone who one day stands on top of that bridge.

Underfoot is the stone deck, walked by human feet day after day. Alongside runs the row of railings, still new, many times younger than the bridge. And if you bend down to look through one of the four holes in the shoulders of the arch, you will see the path a stonemason planned for the floodwater fourteen centuries ago.