Local people refer to the bridges that seem to rear up abruptly from their abutments and then soar dramatically cross steep chasms as “centipede bridges” because of their resemblance to the arch-like rise of a long arthropod’s body as it crawls. From a distance, these bridges appear to be supported by a type of wooden arch, but in actual fact it is an illusionary “arch” that emerges from the interlinking of a series of logs—long tree trunks—that function as interwoven chords or segments of the “arch.” Chinese engineers refer to such a structure as a “woven timber arch,” “combined beam timber arch,” and “woven timber arch-beam” to underscore the use of straight timber members tied together. The basic components are quite simple: two pairs of two layered sets of inclined timbers, with one set embedded in opposite abutments, stretch upward toward the middle of the stream. To fill the gap between these inclined timber sets, two horizontally trending assemblages of timbers are attached. Transverse timbers tenoned to them and/or tied with rattan or rope hold each of the sets of timbers together. It is these warp and weft elements that give rise to the term “woven.”
The downward pressure of the heavy logs compresses all the components together into a tight and relatively stable composition with a significant bearing capacity, the equilibrium can be upset if forces from beneath—such as might come from torrential floods or typhoon winds—push upward. To further stabilize the underlying structure, additional weight is added by constructing an often elaborate building atop the bridge. Somewhat counter intuitively, the heavy timber columns, beams, balustrades, and roof tiles add a substantial dead load that actually increases stability. With the addition of wooden skirts along the side perimeter, the wooden members are then also protected from weathering and deterioration to create a covered bridge. In the West, the covering of a covered bridge is always described as being added in order to protect the underlying wooden structure from weather, and never as added weight to stabilize the structure.
No “rainbow bridge”is more famous than the one depicted in Zhang Zeduan’s twelfth-century celebrated Qingming shanghe tu scroll, a section of which portrays an interlocked arch of piled beams permitting even large vessels to pass beneath its humpbacked opening. Along the surface of the bridge as well as in nearby lanes are busy markets. © Palace Museum, Beijing.
In 1999, in Jinze, a watertown suburb of Shanghai, a modest “rainbow bridge”was built as part of the NOVA television series to re-create the wooden substructure of the famed twelfth-century Qingming shanghe tu using ”woven” timbers to form an arch-like interlocking structure.
In 1999, an American television crew associated with the science series NOVA worked with a team of Chinese scholars and timber craftsmen to design and build a Chinese bridge. Believed at the time to be attempting something unknown except in a twelfth-century painting, the project successfully completed a model bridge that still stands in Jinze, a canal town on the shores of Dianshang Lake in the western suburbs of Shanghai. Working without plans and increasingly aware of how difficult the tasks were, the team experimented with materials and techniques to create an interlaced superstructure of beams placed under and over girders that meta-morphosed from a beam to an arch structure. Their bridge is a modest one when compared with those still found in southern Zhejiang and northern Fujian or even the one depicted by Zhang Zeduan.
Far surpassing this modest effort to create a rainbow bridge reminiscent of the one in Kaifeng are the numerous covered bridges with similar structures in Zhejiang and Fujian provinces, details of which are given on pages 218–47. A fine example, not discussed there, is Xianju (Home of the Immortals) Bridge, about 20 kilometers from the Taishun county town. Built first in 1452 and then rebuilt in 1673, it has the longest span of any bridge of this type in Taishun—34.14 meters—and an overall length of 41.83 meters. The bridge was spared when the highway was improved with an adjacent modern bridge. Rising like a slithering centipede, the covered gallery of the Xianju Bridge utilizes eighty slender pillars to fashion its nineteen internal bays. The rooftop ornamentation is especially notable.
Rising high enough for motorized vessels to move beneath it, Jinze’s newly constructed timber “rainbow bridge”joins many very old stone bridges along the nearby canals.
Covered Bridges
Many of the covered bridges in China are not rainbow bridges because they have underlying supports that differ from the woven timber arch-beam structure. The genesis of covered bridges in China, with traditions that predate covered bridges elsewhere in the world, is quite varied and the forms that are still seen are strikingly different from those that occur in Europe and North America. Covered bridges emerged in Europe in the fourteenth century, principally in mountainous areas in Bulgaria and Switzerland, before subsequently being built throughout the continent. While the few covered bridges remaining today in Europe are individually distinct and historically important, they generally do not have the aura of romance and nostalgia that wafts about covered bridges in North America. In the United States and Canada, covered bridges became common only in the first decade of the nineteenth century, subsequently becoming iconic elements of American rural and urban landscapes when horse-drawn vehicles dominated. Using patented truss designs, they proliferated until 1855 when the introduction of improved steel alternatives led to a dramatic drop off in their construction. Over the years, fire, flooding, vandalism, rotting, and overloaded vehicles swept away thousands of wooden covered bridges. Today, fewer than a thousand covered bridges remain of the 14,000 once standing in North America, where wooden covered bridges are universally recognized as worthy of preservation and valued as emblems of times past. Today, it is estimated that at least 3,000 covered bridges are in existence in China, a number that far exceeds those elsewhere in the world, and are among the oldest structures still standing. Yet, old bridges continue to be lost due to floods, typhoons, vandalism, fire, and replacement by modern bridges to meet current needs. It is often difficult to spot the ruins of old covered bridges because timbers and stone are usually quickly scavenged for use as building materials elsewhere.
While old photographs of bridges in China frequently pointed to the existence of small structures atop even single-arch stone bridges or to pavilions along longer bridges, rarely were large ostentatious structures photographed, probably because they were not encountered. Indeed, until very recently long covered bridges in China were essentially unknown outside China. Yet, whether the covered housing sits atop several stone columns, a series of brick or stone arches, a cantilevered wooden structure, or a “woven timber arch-beam,” it is now apparent that the Chinese constructed some of the most complex, most ornamented, and most beautiful covered bridges in the world. The renovation, rebuilding, and new construction of covered bridges in China have been increasing in recent decades. While many of these various types will be extensively discussed on pages 218–61, some will be introduced here.
Most covered bridges in China are constructed in exactly the same fashion as local houses and temples, using timber frame construction and a conventional set of elementary parts. The superstructure of the bridge serves as the “foundation,” with the floor of the bridge being paved with bricks or stone or overlain with sawn timber. Most covered bridges are I-shaped structures that are comprised of the number of bays necessary to span a particular distance. Often, as with land-based structures, the number of bays is an odd number since such numbers are considered auspicious. Wooden benches, some quite elaborately made, usually run along the full length of any covered bridge. Some of these covered bridges are analogous to roadside pavilions, differing only in that they span a body of water.
The covered corridor of the Santiao Bridge, Taishun, Zhejiang, is reminiscent of the wooden framework of common houses