Source: CCTV.com
11-30-2008 18:20
 |
Visitors to London who take a walk along the banks of the River Thames rarely fail to remark on the bridges. Probably the most striking is Tower Bridge, with its unique and grand Gothic design. The main structure is supported by four towers, two small ones on the banks and two major towers standing on piers.
Another of the world’s famous bridges is the Golden Gate Bridge. One of the most striking scenes in movie history – the aerial fight scene at the climax to the James Bond film “A View to a Kill” – was filmed at this major San Francisco landmark.
 |
In countries around the world, bridges are a unique feature of the landscape. A trip around the world to see all the greatest and most impressive bridges would no doubt be a fascinating journey.
Bridges are found in numerous different locations; across valleys and rivers, over lakes and seas. Not only are they passages across natural barriers, but also they represent milestones in the human transformation of nature.
Such a milestone was established over 1,400 years ago in China, when the very first stone arch bridge in the world – the Zhaozhou Bridge – was built in Hebei Province.
Still today, the Zhaozhou Bridge, at 64.4 metres long and with a span of 37.5 metres, remains one of the longest stone bridges in the world. In September 1991, the American Society of Civil Engineers placed it at Number 12 on its list of “historic civil engineering landmarks” . The list included such famous historical sites as the Pyramids in Egypt, and the Leaning Tower of Pisa in Italy.
As one of the oldest bridges in the world, the Zhaozhou Bridge was the source of numerous great innovations.
The Zhaozhou Bridge was originally called the An Ji Qiao, or Safe Crossing Bridge. Locally it was known as the Da Shi Qiao, or Great Stone Bridge. It lies across the Xiao River in Zhao Country, Hebei Province, and was built over 1,400 years ago.
During the Sui Dynasty at the end of the 6th and the beginning of the 7th century, Zhao County was known as the “northern crossing” , because it occupied a militarily and strategically important position. The Xiao River, which ran across Zhao County, was a key transport artery. However, the river was prone to flooding every summer and autumn, and this made it impossible for ships to pass. So people sought out alternative transport routes.
The Sui authorities decided to help the local people by building a bridge over the Xiao River. The project was entrusted to a group of engineers, led by a man named Li Chun.
The first issue they had to tackle was, what the bridge should be made of.
Traditionally, there were three options. One was wood, but wood is prone to bending, which made it suitable for bridges over slow rivers with light traffic. Another option was a suspension bridge, held in place by iron chains on either bank. But it wouldn’t be stable enough for carts to pass. The final choice was stone. Usually built in a semi-circular shape, a stone bridge was the most solid option.
A stone bridge would be strong enough to bear carts and wheelbarrows. So it was the ideal choice for the Xiao River, with its heavy traffic. There was another advantage; the local mountains were a rich source of large rocks. So Li Chun decided that the bridge should be made of stone.
However, the Xiao River is not only fast-flowing, but also wide. Even at the slowest, narrowest point, the bridge would still have to span 37 metres. Fourteen hundred years ago, no bridge on such a scale had ever been built before.
In a stone bridge, the heavy stones rest on piers. The wider the span, the more piers are needed. A bridge spanning 37 metres would have to describe a series of semi-circles. But this would require building so many piers in the river that a danger would be posed to passing shipping.
So, the idea was proposed of creating one large semicircle.
But there was a major problem. According to the principles of mechanics, the bridge would have to be built to an incredible height of 2,149 metres, the equivalent of 14 pyramids. This was clearly impossible. Even if it could be built, it would be an insurmountable mountain, rather than a help to the local people.
Li Chun, in order to solve the problem, came up with several innovative design innovations.
The first innovation in the Zhaozhou Bridge: The shape of the arch.
The Zhaozhou Bridge’s arch, in spanning 37.02 meters, completes less than half of a semicircle. Just 7.25 metres high, it stretches between the banks like a gigantic rainbow.
Li Chun found the inspiration for the design in Nature. One day, after a rain shower, he saw a rainbow stretching like a bridge between two mountains. This gave him the idea for perhaps the boldest innovation in his bridge design.
However, Li Chun had only limited scientific knowledge, so he could not be certain that his rainbow-shaped arch would support the bridge’s weight.
In Chinese martial arts many gestures describe an arc. An example is the horse-riding stance in traditional boxing. The bent legs cancel out the impact of an attack without affecting the body’s overall stability. Since bending the legs could ensure stability, it must have seemed logical to assume that the principle could be applied, to make a bridge stable.
n experiment shows whether there is any scientific basis for the hypothesis.
An eggshell with its edges smoothed is placed upside down on a table, so that it points upward, just like the Zhaozhou Bridge. Then a nail is dropped on the eggshell from a height of 10 centimetres.
The eggshell, though thin, is undamaged. According to the principle of dissipation in mechanics, the arch spreads the downward force in various directions. The same principle is also applied in the design of helmets.
Although the rainbow arch offered the potential to build a low bridge with a wide span, it remained doubtful if it could withstand heavy traffic passing over it for any length of time.
With this in mind, Li Chun came up with another innovation in bridge design.
The second breakthrough in the Zhaozhou Bridge was the open spandrel.
Fashion designers are familiar with open spandrels. The bat-like shirt that once enchanted the world looks exactly like the two ends of the Zhaozhou Bridge. Take the bridge as a human body, and its two ends are like the shoulders. The two ends, resembling doors left ajar, are known as open spandrels.
The Zhaozhou Bridge describes not only one long and beautiful rainbow arch. It also incorporates four small arches. The four symmetrical arches are typical of open-spandrel bridges. Though small, these arches have played a vital role in ensuring that the bridge has remained spanning the river for over 1,000 years.
The introduction of these small arches helped solve three major difficulties.
The first difficulty was related to the weight of the bridge.
Reducing the weight of the bridge, and thereby the force it exerted on the piers, represented a huge challenge. The local stratigraphic conditions added to the difficulties.
The geological conditions in the Zhaozhou Bridge area have been heavily influenced by the long-term flow of the river. The land surface is rough sand. Below the surface are fine pebbles, rough stones, fine sand and clay. Such a land composition is capable of bearing only between 4 and 6 kilos a square centimetre.
Yet, astonishingly, the Zhaozhou Bridge stands on this sandy base, without pilings or special supports. Its piers consist of nothing more than 5 layers of ordinary stones. Though the structure is simple, it has proved solid and enduring.
The weight reduction is achieved by the four small arches. The two bigger ones have a span of 3.8 metres, and the two smaller ones, 2.8 metres. Creating these hollows saved 180 cubic metres of stones, which represented a weight of over 700 tons, or 15% of the bridge’s overall weight. Accordingly, the pressure on the ground is 5 to 6 kilos a square centimetre, which is within its tolerance range.
After 1,000 years, the bridge’s piers have sunk by a mere 5 centimetres.
The second difficulty, related to the bridge’s stability.
Although the four arches reduced the bridge’s weight, the question was: Would they also reduce its stability?
In fact, the four arches were designed not only to reduce the weight of the bridge, but also to distribute the force it exerted.
The bridge’s weight is first directed to the small arches, where it is spread in three directions: to the piers, to the two banks, and to the main arch. Thus the pressure on the main arch is eased, and this reduces the propensity for distortion and improves the bridge’s stability and ability to bear weight.
Inside each of the small arches is a thin 16 to 30 centimetre layer of stones. They serve to disperse the pressure and distribute the force evenly, thus preventing the concentration of weight on specific points.
The third difficulty was, allowing the release of floodwaters.
Every summer and autumn when the floods come, the four arches become a channel for the waters to pass. Though small, they are calculated to increase the flood release area by 16%. This significantly reduces the impact of the floodwaters on the bridge.
According to hydrological records, the Zhaozhou Bridge has withstood more than ten major floods over the centuries. The most severe was in 1963. The water reached the top of the arch, so that the whole bridge was virtually submerged, Amazingly, though, the bridge survived.
All-in-all, Li Chun demonstrated a remarkable mastery of mechanics, hydrology and geology. It seems clear that he inherited a wealth of scientific knowledge and experience from his predecessors.
Much later, similar designs appeared in Europe. The Antoinette Bridge at Vielmur-sur-Agoût in France, was followed by a large stone bridge in Luxembourg. They marked the beginning of the European tradition of wide-span open-spandrel bridges. Yet, by that time, the Zhaozhou Bridge had been in use for over 1,200 years.
With its main arch and symmetrical small arches, the Zhaozhou Bridge resembles a quarter moon resting on the river. Though made of cold stone, it enhances the landscape. But how were the many stones assembled?
It may be hard to believe, but the stones are joined not by clay or cement, but by iron chains. In fact, the main body of the bridge comprises 28 vertical iron chains and numerous stones.
This is the third innovation incorporated into the Zhaozhou Bridge. In architectural terms, the method is called transversely-bonded arches.
From under the bridge, it is possible to see lines of arches arranged side by side like fingers. This is the key feature of transversely-bonded arches.
The Zhaozhou Bridge is 9 metres across, which was unusually wide for the time it was built. 28 arches of curved limestone slabs were needed to support this width. These 28 arches, each formed of slabs 35 centimetres wide, stand side by side to form the skeleton of the bridge.
This structure, by guaranteeing the width of the span and reducing the height-to-span ratio, significantly increases the bridge’s weight-bearing capacity. It was built slab by slab, a method that saved considerably on labour and materials. An additional feature is that the slabs stand independently of each other, bearing their own share of the weight, so that even if one of them fails, the bridge as a whole won’t be affected.
Just to make sure that the bridge was completely safe, Li Chun added three safety guarantees.
The first guarantee was the stones. Rather than being square in shape, they were wider at the bottom, so that they inclined inward when they were assembled. This is why the bridge’s width decreases in the middle compared to the two ends, from 9.6 metres to 9 metres. The inward inclination prevented the slabs from tilting outward.
Besides, the slabs are finely textured on their sides, which increases the friction between them and their transverse connections.
The second guarantee is found in the iron dovetails. They are installed between the stones to join them tightly together. These iron dovetails bind the 28 slabs into an organic whole.
The third guarantee is the 9 iron joists that run through the bridge. They serve to integrate the 28 slabs. Even the 4 small arches have their own iron joists, serving a similar function. With them, the stones really become a complete whole.
The Zhaozhou Bridge is rich in Chinese culture. Its parapets and pillars are carved with bamboo, flowers, dragons and beasts. The most conspicuous of the carvings is that of the flood-taming beast on the top of the main arch. The creature, though fierce in appearance, is well loved, because in Chinese mythology it was said to save people by alleviating flooding.
The Zhaozhou Bridge was built by Lu Ban. Its parapets are decorated with saints. The Elder Zhang Guo crossed it on his donkey and the God of Wealth marked it with the wheels of his cart.
This folk song had been sung for several generations. It refers to a number of legends about the Zhaozhou Bridge and gods. Other works, such as poems and paintings about the Zhaozhou Bridge have also been passed down to us, as, of course, has the Zhaozhou Bridge itself.
The Zhaozhou Bridge is the oldest single-span bridge, and the best-preserved open-spandrel stone arch bridge, in the world. The History of Science and Technology in China, written by the noted British scholar Dr Joseph Needham, lists 26 scientific achievements produced in China between the 1st and 18th centuries. The 18th of them is the open-spandrel Zhaozhou Bridge.
Despite the rapid advances in bridge building technology and the convenience offered by modern technology, the Zhaozhou Bridge, which has withstood the wind and rain for over 1,000 years, remains a remarkable achievement. It is a bridge built by the wisdom of Chinese engineers. It is a bridge that connects past and present, east and west.
Editor:Du Xiaodan
