Friday, March 21, 2014

The Rainbow Bridge

By Alan Kirkwood

This is a little bit of information I researched and wrote talking about the ancient Rainbow Bridge, a bridge that was depicted on a Chinese scroll that was later reconstructed by students of the Massachusetts Institute of Technology Architecture Department in collaboration with a school in China. The concepts of this bridge can now be seen in many bridges today. It was most known for its little need for nails and bolts but still having great strength due to its interwoven parts.

The Rainbow bridge was an ancient structure of China during the Song Dynasty and was depicted on Qingming Scroll. This scroll has imagery on it showing one of the festivals during this time in the 12th century. It is considered one of the most famous works of Chinese history. The bridge is located in the center of the scroll and appears to be almost floating over one of the town's canals. In the image so many townspeople and vendors are shown crossing it and there seem to be no visible supports. There is no trace of this bridge today outside of what is shown on the scroll and has remained a mystery, studied by many scholars.

In 2000, professor Tang Huan Cheng, who had been studying the bridge for years, made plans to replicate the bridge but limited himself only to what materials would have been available at the time. He joined with a group of students from MIT that were entering into the same pursuit to help execute his recreation of the bridge. They sought out to construct the interlocking bridge with only ancient nails, bamboo ties, and massive abutments and were able to build it after several attempts. They built it over a small canal in Jinze and it was extremely strong. There were two groups of timbers, one having an extra timber than the other, in the longitudinal direction. There were larger members in the transverse direction that helped increase the friction making the bridge impossible to move or damage. They pressed against the longitudinal beams. "The structure in general is an arch structure; the longitudinal members were dominated to press forces. However, not exactly as stone arch, the transverse beam acted on the middle of the longitudinal one will create bending moment in it. At the same time, timber has a great tensile strength than stone material so it is reasonable to carry some bending moments in timber arch." (Y. Yang, Timber arch bridges in China)

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