Countless items that we take for granted in modern life originated in ancient China, from paper and printing, silk to gunpowder, kites, and sophisticated medicine and surgery. However, the Chinese exhibited their greatest skill and ingenuity in the creation of tens of thousands of bridges that were vital in unifying their diverse land. Two thousand years ago, their architects developed iron suspension bridges and daring arch designs that had no rivals in the West until the coming of the industrial age. Whether spanning a yawning gorge or crossing a placid canal, they were masters at integrating function and aesthetics in their bridge construction…
In 1999, NOVA, the acclaimed science series on US channel PBS, explored the wealth of China’s bridge heritage. Their experiments focused on an ancient design that baffled engineers and scholars—the Rainbow Bridge. A ‘blueprint’ derived from the renowned 12th century Qingming Scroll painting that depicts life in the Song Dynasty capital of Kaifeng around 1100 A.D. It is a panorama that teems with more than a thousand tiny figures bustling about their daily life and involved in weddings, funerals, and war games. At the centre of all this activity is the Rainbow Bridge. Lined with shops on both sides, the bridge is an avenue for food hawkers, peasants and street peddlers.
One Song Dynasty historian wrote, “The bridge has no piers, but giant timbers spanning the void, decorated with red paint and curved like a rainbow.” The design is neither an arch nor a beam, but rather a delicate hybrid of the two; a series of interlocking horizontal and cantilever beams that form a graceful arc. It was a style never attempted in the Western world.
How could such a daring and intricate structure have been assembled above a river? Retired engineer Tang Huan Cheng knew the answers. He has carefully studied the Rainbow Bridge for nearly fifty years; now NOVA helped him fulfill his long-cherished dream of reconstructing it. Starting from scratch, Professor Tang is joined by a team of experts who devised a plan for the bridge based on scant historical information and a close analysis of the painting.
Bashar Altabba, a structural engineer who teaches at MIT, and Marcus Brandt, a timber framer from Pennsylvania, got together at MIT to work on a model for the bridge. They experimented with a method of prefabricating the skeleton of the bridge and then lowering it together from either side of the river. The NOVA team of engineers, bridge historians, and construction experts went to the town of Jinze, not far from the famous Song city of Suzhou, to reconstruct a Rainbow Bridge. The team must build strong abutments that will prevent the bridge from sinking into the river’s soft muddy banks. The first job was to build a dam.
Woven strips of bamboo are put into place to give the dam shape. The men shovel in clay to make a watertight barrier. When this dam is complete, water was bailed from the space where the abutment will be built.
The men pound 36 piles a full six feet into the sub-soil. Over the next week, workers lay 15 tons of stone on top of the pilings. However, Altabba worries that the labourers have not stacked the stones in a way to optimize the foundation’s strength. The men work to fix the abutments so they will stand strong when pressed by the weight of the bridge. The builders have moved the abutments so they don’t have to cut down a mature tree. However, the new placement doesn’t leave enough room for the prefabricated elements on the riverbanks.
This means the team had to abandon the plan of lowering the two sides of the bridge from either bank. Instead, the men build the bridge’s arch while perched on two boats in the river. One of the unique features of the Rainbow Bridge is the interlocking arch made of 64 wooden logs and 5 transverse beams.
Workers weave in one board after another to create an arch that is as intricate as it is strong. The construction crew uses nails to hold the structure together in some places, and bamboo rope in others—two methods that the ancient Chinese could have employed. To make rope pieces that are long enough for proper lashing, the team sets up a rope-making device. With the arch structure of the bridge in place, the men now put down planks.
Step by step, the Rainbow Bridge came to life, 16 metres long and 4 metres wide.
Professor Tang organized a demonstration to show that he has full faith in the bridge’s strength. His men guided two water buffalo onto either end of the bridge. Their combined four-ton weight didn’t budge the bridge’s arch.
In the evening, the people of Jinze watched a firework display from the perfect spot in town: their new Rainbow Bridge.
Professor Tang, happy to see the realization of his life-long dream, observes: “Bridges have always helped bring people together. You can see it today. People from both sides of the river are now more closely connected. But the rainbow arch is also a bridge across time, from the present to the past.”
With thanks to:
Secrets of Lost Empires: NOVA builds a Rainbow Bridge First broadcast on PBS, 1999
Chinese Bridges: Living Architecture From China’s Past by Ronald Knapp
Chinese Ancient Timber Arch Bridges by Dr. Jie Liu