Some Covered Bridges

My selection here isn’t intended to be exhaustive. Bridges are covered for all sorts of reasons but usually for a practical one. In northeast USA for example, there’s the Cornish-Windsor Bridge between New Hampshire and Vermont. It was built in 1866. Pedestrians can walk across but at some risk as the bridge is only wide enough for two cars to pass. This wasn’t a problem in 1866. Covered bridges such as this can have a lifespan of 100 years or more, as opposed to uncovered ones that typically have a lifespan of about 20 years because of the effect of rain and snow on the structure. This is understandable, especially in places where the temperature is below zero at night and above zero during the day for extended periods twice a year. The alternating freezing and thawing of water hastens the weathering and failure of even masonry and concrete structures. This bridge is located in such a place.

Whatever the climate, it’s always good to keep rain off timber as water doesn’t have to be freezing to destroy wood. This next bridge is the Wenxing Bridge in Taishun County in China’s Zhejiang Province that, along with neighboring Fujian Province have a number of similar covered bridges. Many people think Wenxing Bridge the most beautiful covered bridge in China. Zhejiang and Fujian Provinces both have a warm-temperate bordering on sub-tropical climate with significant rainfall and mountainous areas. It’s not uncommon for villages to be split by a river and so the bridge becomes a place to go to to avoid the heat or the rain, for buying and selling things, and just to hang out with people from the other side. Wedding and other commemorative photographs are taken.

Bridges of this type have vertical timbers protecting the structure below and, last week when I visited, I wish I’d remembered to see how they were connected as the bridge is said to be constructed without any nails. I did notice some of the upper columns had metal straps around them to prevent cracks developing into splits. It’s only just visible in the photograph above but the main support structure consists of rows of interlocking logs. The main structure is as shown below. The logs are held in place by gravity alone. No nails, screws, straps, rope lashing or clever joinery. The second roof at the centre of the bridge is largely decorative in that there are bridges without them, but it does add additional weight to further stabilize the sub-structure timbers.

In September 2016, the level of surging typhoon-driven floodwaters rose to the level of those structural timbers and the bridge was swept downstream where the pieces were later collected, brought back upstream and the structure reassembled over the next year and two months. The same happened to two other nearby bridges. There are variations on the form and ornament but, for longer spans, the basic structure is repeated to span anything from one to five piers.

These bridges are UNESCO World Heritage items so there’s no lack of information. You can find more images and information HERE.

This is the Chenyang Wind and Rain Bridge (程阳风雨桥, Chéngyáng Yǒngjì Qiáo) in Guangxi, China. It was built in 1912. The total length is 77.76 meters and the four spans are 17.3m wide. The pavilions aren’t just being pretty – they’re essential to the safety of the structure because they add weight to counter the leverage of the timbers cantilevered from the piers below. A big box of rocks would do the same thing but maybe somebody decided that five tiled roofs would do the same thing and be much nicer to look at. This makes sense, whereas building masonry piers to cantilever timbers and then put masonry above them does sound a bit iffy, even if it makes sense as a force diagram.

In October 1965, the poet Guo Moruo named the bridge “Chengyang Bridge” and composed a seven-character quatrain:

“艳说林溪风雨桥，桥长廿丈四寻高。
“重瓴联阁怡神巧，列砥横流入望遥。 
“竹木一身坚胜铁，茶林万载茁新苗。
“何时得上三江道，学把犁锄事体劳。”

“The Linxi Wind and Rain Bridge is beautiful, twenty feet long and four fathoms high.
“The double roof and pavilions are ingenious and delightful; the water flows into the distance.
“Bamboo and wood are stronger than iron; tea trees have been growing for thousands of years.
“When will I be able to go up to the Three Rivers and learn to plow and hoe and do hard work?”

The wood structure is well above water level and has layers of protruding timbers that provide cantilevered support for the bridge proper along which various souvenir items are now sold. The limits of cantilevering logs have been continuously studied since the 4th century and so, each time a bridge has to be rebuilt, it can be rebuilt using shorter logs. Chenyang Bridge uses logs of 7–8 meters to span 17.3 meters. Most of the Chenyang Bridge that we see now is from a reconstruction from a 1982 flood. It’s still the same bridge.

There are five levels of roof but the lowest one is a more of an awning to deflect water from the important timbers below. The pavilions are pretty yes, but they also have the important function of adding weight to counterbalance the turning moment of the cantilevers. The more compressed the logs, the stronger the structure. A similar result could have been achieved by having a masonry structure above the piers but, for one, it wouldn’t make sense to change from a masonry structure to a wood one and then back to a masonry one. The additional timber and tile roof structures are known structures and far prettier than an equivalent weight of rocks in a box. I’m sure that height of pavilion probably has something to do with width of span but I’m also sure this knowledge isn’t recorded but handed down between bridge builders. Bridge building isn’t a dying art. Rebuilds aside, about three or four new ones are completed each year.

Here’s one I only know about from seeing a photo of it in a restaurant. Fuqing Bridge (福庆桥) is a very strange animal, perhaps because the masonry piers have been replaced by eight columns that support each end of the covered wooden arches with woven beams. The original bridge (left, below) was built in 1880 but destroyed by a typhoon in 1990. The new bridge (right) is 170 meters long, 4.8 meters wide, and 12.7 meters above the water. Reconstruction began in 2015 and was completed in 2019. It is the longest wooden arch covered bridge in China and has fifteen rooms, a Buddhist Hall and a three story pavilion in the centre.

The new bridge shares some DNA with the one it replaces .The concrete frame is the base for the timbers as well as supporting the flat parts of the bridge as the original masonry piers did. There are still three spans with the middle one being the largest. These span lengths look like they’re about the limit for this type of structure. Replacing the flat piers with concrete columns is probably better for flow and discharge when the river is swollen with typhoon rains. It looks like they’ve planned for a river level rise of about 6 meters. It’s great that the parts of a destroyed bridge can be collected and reassembled but there’s a balance between maintaining tradition and having other things to do. The former bridge hasn’t been simply rebuilt out of nostalgia. It still resembles the former bridge but the slim concrete columns don’t obstruct the flow of the river as much as the former masonry piers did. It’s not the Chinese way to think they can improve upon tradition but bridge building is a living tradition. Bridges such as these are a response to geographic and climatic conditions and are of course influenced by transportation needs as well as social and cultural practices. The lesson was learned the hard way but this new bridge continues that long tradition by acknowledging the link between bridge longevity and river hydraulics.

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People in China like bridges. These covered bridges are just one type that people are particularly attached to. You will sometimes see photos of them next to or downstream from masonry traffic bridges that do other things. Chinese people appreciate how these bridges are places for people to meet and not just a way to get to the other side. Villagers who’ve done well somewhere else will often fund the building of a bridge in their village. The building of a bridge is a big thing. These days, not everyone has a village to go back to but urban locations also have rivers and people still need bridges to cross them. It’s for this reason that, uncovered as urban bridges are, they are still the preferred place for evening street-food vendors to park their carts or for Saturday morning market sellers to set out their wares and for local people to maybe meet.

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Further information:

https://www.famousfix.com/list/covered-bridges-in-china