Bridge Technology

Sources

Need. The development of roads and waterways in the imperial era (617-1644) demanded a high level of bridge building. The Chinese constructed different kinds of bridges: timber beam, cantilever timber, suspension, and masonry. In addition, they used the stone arch in many forms: semicircular, horseshoe, elliptical, and segmental.

Cantilever Type. Cantilever bridges of timber and stone existed during the early imperial period. This type of bridge is usually made with three spans: the outer two spans are anchored on the shore, while the third is projected out over the channel and supported by cantilever beams on only one side. A good example of a cantilever bridge existed at Lilin in the Hunan province. It crossed the Lu River and had piers twelve feet wide with a span between them of just over fifty-six feet.

Semicircular Arch Type. By the seventh century, stone had become the typical material for most important

bridges. To solve the problems of building in the sedimentary plain of North China, the Chinese began to develop a lighter and more flexible form of arch. Engineer Li Chun built a semicircular arch bridge in Hebei between 606 and 616, the first of its kind in the world. Named the Great Stone Bridge, it was noteworthy for its four spandrel arches and decorative design. The main arch was composed of 28 parallel lines of 45 massive stones, each approximately one ton in weight and curved in the direction of the span, with a line of thinner slabs on top. Each arch stone was fixed to its neighbors in the direction of the span with two iron clamps. The two pairs of segmental arches in the spandrels not only served to reduce the load of the masonry on the haunches of the arch but also provided an overflow for floodwater. The bridge had a span of 123 feet. Later similar bridges were built with flat segment arches and open spandrels. In these cases the arch stones were relatively flat and shallow, often longer in the direction of the span than in the direction of the center of the curve radius. Therefore, they did not rely on their own heaviness for stability, instead fitting together exactly to shape a thin shell, which passed the load on to the abutments. Mortar was not used in the arch and foundations, and the masonry foundations were often set on wood piles.

Segmental Arch Type. During the imperial era, a break-through in bridge construction happened when Chinese engineers discovered that a bridge could be built based on a segmental arch rather than the traditional semicircular arch. Segmental arch bridges, using less material, were physically more powerful than ones constructed with semi-circular arches. The bridge in the Song era (960-1279) painting By the River at the Qingming Festival was a typical segmental arch bridge. It had parallel rows of five round section beams overlapping each other. Five rectangular transverse beams were placed between the arch beams so that the whole interlaced structure became a rigid flat arch in timber. A layer of dirt covered the boarded deck.

Marco Polo Bridge. The greatest segmental arch bridge in China is the well-known Marco Polo Bridge, frequently so called because Polo illustrated it in detail. Located just west of Beijing and constructed in 1189, it is still in use today. The 771-foot-long bridge is composed of a series of 11 segmental arches (each with an average span of 62 feet) extending one after another across the Yongding River.

Suspension Bridge. Most of the suspension bridges were in southwestern China, especially in Sichuan and Yunnan provinces, where there are many deep river gorges. The Chinese began to build chain suspension bridges as early as the seventh century, using twisted bamboo cables about four inches thick. (The cables were replaced every year.) One suspension bridge in Sichuan, for example, had ten parallel cables and five more cables on each side, one above the other, shaping the balustrades. The total length of the bridge was about one thou-sand feet. It consisted of nine unequal spans, the largest of which was about two hundred feet. There were seven intermediate wooden towers and one stone-built tower close to the center of the bridge.

Transverse Shear Wall. During the Song dynasty the Chinese developed the transverse shear wall to build more-slender and flexible stone bridges. This solid wall at right angles to the span was set close behind the spring of the arch on both sides of the watercourse, reinforcing the abutment with its relatively deeper foundations. In addition, for purposes of economy, only the sidewalls of the bridges were built of masonry, with the remainder of cheaper materials such as rammed soil. This wall also reduced the effects of unequal settlement between the main foundations and those of the approach.

Sources

Derk Bodde, Chinese Thought, Society, and Science: The Intellectual and Social Background of Science and Technology in Pre-Modern China (Honolulu: University of Hawaii Press, 1991).

James M. Hargett, On the Road in Twelfth Century China: The Travel Diaries of Fan Chengda (1126-1193) (Stuttgart: Steiner Verlag Wiesbaden, 1989).

Joseph Needham, Clerks and Craftsmen in China and the West (Cambridge: Cambridge University Press, 1970).