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The Genius of China3,000 Years of Science, Discovery, and Invention
By Robert Temple
Inner TraditionsCopyright © 2007 Robert Temple
All right reserved.
from Part 3
The Suspension Bridge
First Century A.D.
Few structures seem more typical of the modern world and its engineering achievements than the suspension bridge. And yet, the sophisticated form of the suspension bridge, with a flat roadway suspended from cables, was unquestionably invented in China. And it is highly likely that the two more primitive forms of suspension bridge also originated there, the simple rope bridge and the catenary bridge (where the walkway or roadway is not flat but follows the curve of the cables).
The simplest form of ‘suspension’ bridge--if we can even call it that--is simply a rope thrown across a gorge. Probably from the very beginning, the technique used for getting the rope across was that still used later for elaborate suspension bridges--shooting it across, tied to an arrow. After the Chinese invention of the crossbow greater power would have been available for heavier cables over longer distances.
Climbing or scrambling along a single rope above a gorge can be dangerous, and is hard on the hands. An ingenious solution is still in use in some areas, such as the Tibetan-Chinese border. The rope is threaded through a hollow piece of bamboo before being attached, and the person merely hugs the bamboo and slides along the rope without burninghis hands or straining himself unduly. A more sophisticated method is by a cradle attached to the bamboo tube. Cable bridges of liana vines are known in the Andes mountains of Peru, dating back to at least 1290, and Needham suspects that this may be one of the many Chinese ideas to have spread to the New World across the Pacific.
Bridges of ropes and cables in China and Tibet evolved into multiple-cable bridges of various types. Sometimes three ropes or cables are stretched across together so that the person crossing can walk with his feet on two of them and hold a third above his head for balance. Or a woven walkway of matting is incorporated between the two bottom ropes or cables, to make the going easier. Another variation is to have a series of hanging straps by which the user pulls himself forward. All these and other variations occur in the area between China and Tibet, in the high mountains. A reference in the Chinese dynastic history for 90 A.D. appears to mention a suspension bridge which has planking and, hence, a proper platform upon which to cross:
There the gorges and ravines allow of no connecting road, but ropes and cables are stretched across from side to side and by means of these a passage is effected.
Th is reference is rather vague. The same dynastic history for 25 B.C. describes a harrowing Himalayan suspension bridge:
Then comes the road through the San-ch’ihp’an gorge, thirty li long, where the path is only 16 or 17 inches wide, on the edge of unfathomable precipices. Travellers go step by step here, clasping each other for safety, and rope suspension bridges are stretched across the chasms from side to side. After 20 li one reaches the Hsien-tu mountain pass. . . . Verily the difficulties and dangers of the road are indescribable.
Fa-Hsien, the first Chinese Buddhist pilgrim to India, crossed this very bridge in 399 A.D. and left this account of his experience:
Keeping on through the valleys and passes of the Ts’ung-ling mountain range, we travelled south-westwards for fifteen days. The road is difficult and broken, with steep crags and precipices in the way. The mountain-sides are simply stone walls standing straight up 8000 feet high. To look down makes one dizzy, and when one wants to move forward one is not sure of one’s foothold. Below flows the Hsin-t’ou Ho. Men of former times bored through the rocks here to make a way, and fixed ladders at the sides of the cliffs, seven hundred of which one has to negotiate. Then one passes fearfully across a bridge of suspended cables to cross the river, the sides of which are here rather less than 80 paces [400 feet] apart.
Cable bridges in China were most efficient when made of bamboo. The cables were made with a centre formed of the core of the bamboo surrounded by plaited bamboo strips made of the outer layers of the wood. The plaiting was done so that the higher the tension, the more tightly the outer strips gripped the inner core. This led to the safety factor that it is the inner strands of a cable which snap first, rather than the outer strips which would otherwise unravel very fast. An ordinary 2-inch hemp rope can stand stresses of only about 8000 pounds per square inch, but bamboo cables can stand a stress of 26,000 pounds per square inch. Ordinary steel cables will only take twice as much stress (56,000 pounds), so bamboo is remarkably strong. (Modern steel alloys such as used in the Golden Gate Bridge at San Francisco can take stresses of 256,000 pounds per square inch.)
The most famous Chinese suspension bridge is a catenary bridge (which has a roadway following the curves of the cables rather than hanging flat): the An-Lan Bridge at Kuanhsien in Szechuan. It has a total length of 1050 feet, composed of eight successive spans, and there is not a single piece of metal in the entire structure. An account of a traveller crossing it in 1177 describes only five spans at that time. It has planking on which to walk, originally 12 feet wide but today only 9 feet wide, and it is believed to have been built in the third century B.C. by Li Ping.
Excerpted from The Genius of China by Robert Temple Copyright © 2007 by Robert Temple. Excerpted by permission.
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