Bridges sometimes are accidental configurations—a mere log or bamboo thrown over a channel as well as stones deposited by flood across the breadth of a stream—but more often they represent purposeful efforts to tease strength from materials in order to overcome a gap in space. Until recent centuries, bridge building anywhere in the world was a more practical and empirical art than engineering science. Primitive responses using common materials such as wood, stone, and rope of many types progressed over time into more complex solutions using exactly the same materials. Long before science and mathematics were consciously employed to design bridges, local craftsmen experimented with easily accessible materials nearby to solve their bridging needs. Yet, whatever the level of sophistication of the builder or designer, each was forced to consider fundamental and common realities that tested the nature and limits of various forms of matter. In addition, as elsewhere in the world, opportunities emerged in China for exploring and ultimately devising innovative methods of prevailing over old problems with art and grace using a rich range of technical solutions, many of which actually predated similar advances in the West.
Slender logs held together by iron staples and laid across a shallow running stream serve here not only as a temporary bridge but also as a convenient place to wash vegetables. Sanjiang region, Guangxi Zhuang Autonomous Region.
The Basics of Bridges
It will be useful here to spell out some of the basic terms used in later discussions since bridge builders in China, like those anywhere in the world and whatever their sophistication, encountered common forms of matter while searching for, perhaps even stumbling upon, inspired solutions. Bridges are differentiated in a number of ways, namely in terms of their materials, structure, and purpose.
Simple bridges made of stone beams are ubiquitous in rural China, such as this stream going through Likeng village, Wuyuan county, Jiangxi. Because stone beams tend to bend due to tension, it is often necessary to support them with midstream piers to increase the distance crossed.Where fracturing occurs, traditional stone slabs are sometimes replaced by prestressed concrete slabs.
Traditionally, there were only three types of building materials—resilient wood, durable stone, and flexible ropes—and three types of bridge structures—beam, arch, and suspension. Cement, iron, and steel, among other later materials, came to offer new solutions to contemporary builders even as they continued to work within the traditional range of structural types and tried-and-true old material types. For the most part, the materials used to build bridges in the past were those that were readily at hand, whether they were timber, stone, or twisted ropes made of bamboo or vines. Yet, as many of the examples shown later in this book reveal, treasured hardwoods, marble, and pliant bamboo somewhat surprisingly came to be used in China in locations far removed from where these materials were commonly found.
The overall configuration of any bridge can be subdivided into its substructure, comprising abutments, piers, and foundations, as well as a super-structure that rises above—beam, arch, suspension, and truss, in addition to any number of combinations of these. Bridge structures differ primarily in the ways in which they bear their own weight, or dead load, as well as the live load—the weight of people, animals, vehicles, even snow. Forces and stresses—tension, compression, torsion, and shear—all impact how successful is the marriage of materials and form in the design and construction of any bridge. In some bridges, these forces act individually, but in most they act in combination, a subject that today is resolved by modeling, but in history was addressed by trial-and-error experimentation.
Tension and compression are opposite forces; the first tends to stretch or elongate something while the second pushes outward. In comparison, torsion is a twisting force, either in one direction while the other end is motionless or twisting in opposite directions. Beside “dead” and “live” loads, the force of the wind, rushing water, and earthquakes can all provide forces that create instability or, in fact, lead to the destruction of any bridge. Arches transfer most of the load they carry diagonally—rather than vertically—to the abutments, pushing outward against these supports rather than transferring their weight downward. In functioning this way, an arch is said to be in a state of compression, with its component parts squeezing outward.
Beams simply rest, with only gravity operating to transfer the weight of the beam and what it carries vertically to the supports on both ends, the abutments, and any midway piers placed to offer supplementary support. Because beams tend to bend at the middle due to downward pull, or tension, perhaps even to the point of breaking, there are definite limits to the length of single beams of different materials.
It is sometimes possible to overcome these limits by linking a series of short beams on intervening supporting piers or by projecting the beams by using cantilevering principles. Trusses, which are a form of complex beams, developed in the West in the nineteenth century, were not a solution experimented with by early Chinese bridge builders.
Since stone is very strong in compression, it is naturally the material of choice for building arch bridges. Sometimes called inverted arches, suspension structures are said to be in a state of tension since they pull inward and away from their anchorages. Used in ingenious combinations, beams, arches, and suspended cables can be employed simultaneously to support a bridge. It is all too easy to overbuild, to waste materials, money, and effort.
Step-on block bridges, where each stone is carved into a similar shape and lined up, are sometimes used to cross shallow streams, as seen here in Shouning county, Fujian.
Elizabeth B. Mock reminds us in her book, The Architecture of Bridges, that successful bridge building results from handling “material with poetic insight, revealing its inmost nature while extracting its ultimate strength through structure appropriate to its unique powers” (1949: 7).
In Europe, bridge building languished for eight centuries after the decline of the Roman Empire, not to be revived until the twelfth century. Famed for their extraordinary engineering feats, including bridge building, the Romans left a tangible legacy of stone works that are rivaled by those in China in terms of dates, styles, and geographic spread. Even exceeding in number those still standing in Europe, many examples of early Chinese bridge building innovations can be found throughout the countryside. H. Fugl-Meyer, a twentieth-century Danish engineer, confidently estimated that perhaps 2.5 million bridges existed in China in the 1920s, with as many as fifty-two bridges per square kilometer in some areas of the water-laced Jiangnan region (1937: 33).
Marco Polo himself noted the seeming ubiquity of bridges in Quinsai or Hangzhou, numbering, he said, some 12,000 bridges, an exaggerated figure that might have been inflated by a copyist.
In China, as in other countries, bridges figure not only in legends and myths but are also detailed as part of the material achievements of emperors down through the ages. It is said that the Sage Emperor Yu at the beginning of the third millennium BC summoned giant turtles to position themselves in a river as a means for him to cross. Early bridge builders, recognizing this precursor technique, used a continuous but broken line of large stones to accomplish the same purpose, forming “turtle bridges,” a type one can still see in shallow streams throughout the countryside.
As early as the twelfth century BCE, Shang dynasty texts began to employ the contemporary Chinese characters for bridge, qiao and liang, both of which include the wood radical, explicitly revealing that wood was a common building material for early bridges. Probably simple logs laid side by side rather than hewn timbers were placed across narrow streams or ditches surrounding settlements, as can be seen in the 7000 BP Neolithic Banpo site near contemporary Xi’an. Limited only by the height of available