The structure, shape and materials of Zhaozhou Bridge

Zhaozhou Bridge was listed as the first batch of national key cultural relics protection units by the State Council in 1961. In 1991, the American Society of Civil Engineers selected Anji Bridge as the 12th international historical civil engineering milestone. Also known as Anji Bridge, it is located on the Xiaohe River in Zhao County, Hebei Province. It is the earliest existing and best-preserved huge stone arch bridge in the world. It has a history of more than 1,400 years and is known as "one of the four treasures of North China". It was built during the Daye period of the Sui Dynasty (605-618 AD) and was built by the famous craftsman Li Chun. The bridge is 50.82 meters long, has a span of 37.02 meters, and a bridge height of 7.23 meters. It is the largest single-hole open-shoulder stone arch bridge with the largest span and the earliest construction in the world today. Because there are two small holes on the shoulders at both ends of the bridge, which are not solid, it is called open shoulder type. This is a creation in the history of world bridge building (those without small arches are called full shoulder or solid shoulder type). There are many things on the bridge, many types and colorful.

The Zhaozhou Bridge was built 1,400 years ago and has experienced 10 floods, 8 wars and multiple earthquakes, especially the 7.6 magnitude earthquake that occurred in Xingtai in 1966. Xingtai is more than 40 kilometers away from here. There was also an earthquake of magnitude 4.0 or so here, but the Zhaozhou Bridge was not damaged. Mao Yisheng, a famous bridge expert, said that regardless of the internal structure of the bridge, the fact that it has survived for more than 1,300 years speaks for itself. The flood in 1963 caused the water to reach the mouth of the bridge arch. According to local elders, you could feel the bridge shaking greatly while standing on it. According to records, Zhaozhou Bridge has been repaired nine times since its completion.

Two small arches are added at each end of the main arch. One is to save materials, and the other is to reduce the weight of the bridge (reduce the weight by 15%), and increase the discharge of river water under the bridge. flow.

In May 1979, a joint investigation team composed of four units including the Natural History Group of the Chinese Academy of Sciences conducted an investigation on the foundation of the Zhaozhou Bridge. The Zhaozhou Bridge has a dead weight of 2,800 tons, and its The foundation is just five layers of stone strips forming a 1.55-meter-high abutment, which is built directly on natural sand and gravel.

Such a shallow bridge foundation is simply unbelievable. When Mr. Liang Sicheng inspected it in 1933, he believed that it was just a diamond wall used to prevent water erosion, rather than a foundation to bear the entire load of the bridge. He wrote in the report:

"In order to actually measure the foundation of the bond, we excavated at the foot of the bond in the north, but about 70-80 centimeters below the current river bed, we found a stone wall lying flat under the bond. The stone wall was found. The fifth floor is 1.58 meters high. Each floor is slightly higher than the one above. There is no solid foundation below. It is clearly just a diamond wall to prevent water erosion, rather than a foundation to bear the entire load of the bridge. . Because water will appear if it goes down 30-40 centimeters, it is impossible to reach the location of the large bridge foundation based on our theoretical speculation unless there is a large-scale excavation."

In September 1991, Zhao The State Bridge was selected as the twelfth "International Civil Engineering Milestone" by the American Society of Civil Engineers, and an "International Civil Engineering Historic Monument" bronze monument was built on the east side of the northern end of the bridge.

In order to protect the Zhaozhou Bridge, the structure of the newly built bridge 100 meters east of the Zhaozhou Bridge at the end of the last century still followed the Zhaozhou Bridge, except that the number of small arches on the main arch increased to 5 on each side.

Design innovation

(1) The use of arc arches has changed the tradition that most large stone bridges in my country are semicircular arches. Most of the arches of ancient stone bridges in my country are semicircular. This form is more beautiful and complete, but it also has two shortcomings: First, the transportation is inconvenient. Semicircular bridge arches are more suitable for bridges with smaller spans, but larger spans. If a semi-circular arch is used for a span bridge, the vault will be very high, resulting in a high bridge and steep slope, making it very inconvenient for vehicles, horses and pedestrians to cross the bridge. Second, the construction is unfavorable. The scaffolding used for the semicircular arch stone masonry will be very high, which increases the risk of construction. To this end, Li Chun and the craftsmen creatively adopted the arc arch form, which greatly reduced the height of the stone arch. The clear span of the main hole of the Zhaozhou Bridge is 37.02 meters, while the arch height is only 7.25 meters. The ratio of arch height to span is about 1:5, thus achieving the dual purpose of low bridge deck and long span, and the bridge deck transition It is stable and very convenient for vehicles and pedestrians. It also has the advantages of saving materials and convenient construction.

Of course, the thrust of the arc-shaped arch on the bridge foundations at both ends increases accordingly, which requires higher requirements for the construction of the bridge foundation.

(2) Use open shoulders. This is a major improvement Li Chun made to the spandrels. He changed the solid shoulder arches used in previous bridge construction to open shoulder arches, that is, two small arches are set up at both ends of the big arch, and the small arch near the big arch foot has a clear span. is 3.8 meters, and the clear span of the other arch is 2.8 meters. This kind of shoulder arch with a large arch and a small arch has excellent technical performance. First of all, it can increase the flood discharge capacity and reduce the impact of floods on the bridge due to the increase in water volume during flood seasons. During the flood season in ancient times, the water potential of the Jiaohe River was relatively large, which was a test for the bridge's flood discharge capacity. Four small arches could share part of the torrent. According to calculations, four small arches could increase the water-crossing area by about 16%, greatly reducing the impact of floods on the bridge. impact and improve the safety of the bridge. Secondly, compared with solid shoulder arches, the open shoulder arch can save a lot of earth and stone materials and reduce the self-weight of the bridge body. According to calculations, the four small arches can save 26 cubic meters of stone and reduce its own weight by 700 tons, thus reducing the impact of the bridge body on the abutment and foundation. Vertical pressure and horizontal thrust increase the stability of the bridge. Thirdly, it increases the beauty of the shape. The four small arches are balanced and symmetrical. The large arch and the small arch form a complete picture, which looks more light and beautiful, reflecting the complete unity of architecture and art. Fourth, in line with the theory of structural mechanics, the shoulder-arch structure puts the bridge in a favorable condition when carrying load, can reduce the deformation of the main arch ring, and improve the bearing capacity and stability of the bridge.

(3) Single hole. Traditional construction methods in ancient my country generally use porous forms for longer bridges, so that the span of each hole is small and the slope is gentle, making it easier to build. However, porous bridges also have disadvantages. For example, there are many bridge piers, which is not conducive to boat navigation and hinders flood discharge. The bridge piers are impacted and eroded by water flow for a long time, and they are prone to collapse over time. Therefore, when Li Chun designed the bridge, he adopted a single-hole long span without piers in the middle of the river, making the stone arch span as long as 37 meters. This is an unprecedented initiative in the history of bridges in our country.

Creativity in construction technology

(1) The bridge site selection is relatively reasonable to make the bridge foundation stable and reliable.

Based on his many years of practical experience and rigorous and thorough investigation and comparison, Li Chun chose a relatively straight place on both sides of the Jiao River to build the bridge. The strata here are formed by alluvial river water, and the surface of the strata is The coarse sand layer that has been washed by water for a long time is followed by fine stone, coarse stone, fine sand and clay layers. According to modern calculations, the ground here can withstand 4.5 to 6.6 kilograms of pressure per square centimeter, while the pressure of the Zhaozhou Bridge on the ground is 5-6 kilograms per square centimeter, which can meet the requirements of the bridge. After the bridge site was selected, the foundation and abutment were built on it. Since the bridge was built, the bridge foundation has only sunk 5 centimeters, indicating that the stratum here is very suitable for bridge construction.

(2) The Zhaozhou Bridge has a novel construction method and is easy to construct and repair.

Li Chun used local materials and selected hard blue-gray sandstone produced in nearby counties as the bridge construction stone. In terms of stone arch laying method, he adopted longitudinal (in the direction of the bridge) laying method. The method is that the entire bridge is composed of 28 independent arches arranged side by side along the width direction. The arch thickness is 1.03 meters. Each arch is independent and can be operated independently. It is quite flexible. After each arch is completely closed, it forms a single bridge. To assemble the coupons independently, after building one coupon, move the "scaffold" that bears the weight, and then build another adjacent arch. This masonry method has many advantages. It can not only save the wood used to make the "scaffolding" and make it easy to move; it is also beneficial to the maintenance of the bridge. If the stones of an arch are damaged, just embed new stones and perform partial repairs. Rather than having to adjust the entire bridge.

(3) Many strict measures have been taken to maintain the stability of the bridge.

In order to strengthen the horizontal connection between each arch and make the 28 arches form an organic whole with tight and firm connections, Li Chun took a series of technical measures. l) Each arch adopts the method of being wider at the bottom and narrower at the top, with a slight "pointing" method, so that each arch is tilted inward and crowded against each other to strengthen its lateral connection and prevent the arch stones from toppling outward; in the width of the bridge A small amount of "squeezing" method was also adopted, which is to gradually shrink the width from both ends of the bridge to the top of the bridge, from the maximum width of 9.6 meters to 9 meters, to enhance the stability of the bridge.

2) On the main bridge, 5 iron tie rods are set evenly along the width of the bridge, passing through the 28 arch bridges. Each tie rod has a semicircular rod head exposed outside the stone to clamp the 28 arch bridges and strengthen it. Horizontal connections. There is also an iron tie rod on each of the four small arches that plays the same role. 3) A layer of arch protection stones is covered on the outer arch stones and the small arches at both ends to protect the arch stones; there are 6 hook stones on both sides of the arch protection stones to hook the main arch stone. The connection is strong. 4) In order to make the adjacent arch stones fit tightly together, "waist iron" is worn between the adjacent arch stones on both sides of the outer arch, and the adjacent stones between each arch are also connected. There is a "waist iron" pierced on the back of the arch to link the arch stones. Moreover, the side of each arch stone is chiseled with fine diagonal grooves to increase friction and strengthen the lateral connection between the coupons. The adoption of these measures makes the entire bridge a tight whole and enhances the stability and reliability of the entire bridge.

(4) The abutment of Zhaozhou Bridge is unique.

The abutment is the foundation of the entire bridge and must be able to withstand the huge horizontal thrust and vertical pressure caused by the decomposition of the axial force of the main arch ring (the main body of the bridge body). The abutment of Zhaozhou Bridge has the following characteristics: 1) low arch foot: the arch foot is only about half a meter below the river bed; 2) shallow bridge base: the bottom surface of the bridge base is about 1.7 meters below the arch foot; 3) short abutment : From top to bottom, gradually thickened stone strips are used to build a bridge abutment that is 5 meters long, 6.7 meters wide and 9.6 meters high. This is an economical, simple and practical bridge abutment. In order to ensure the reliability of the bridge abutment, Li Chun took many corresponding measures to consolidate the foundation. In order to reduce the vertical displacement of the bridge abutment (that is, the sinking caused by the vertical pressure of the main body of the bridge), Li Chun took the measure of driving many wooden piles beside the abutment to strengthen the foundation of the bridge abutment; this method is used in It is also often used in the construction of factories and bridges today. In order to reduce the horizontal movement of the abutment (that is, the backward movement of the abutment caused by the horizontal thrust of the main body of the bridge), Li Chun adopted the method of extending the back seat of the abutment to offset the effect of horizontal thrust. In order to protect the bridge abutment and bridge foundation, Li Chun also set up a diamond wall on one side of the river. On the one hand, it can prevent the erosion of the water flow. On the other hand, the diamond wall is integrated with the bridge abutment and bridge abutment, increasing the bridge's length. The stability of the platform. The above measures ensure that the bridge has a solid abutment and improve the solidity of the bridge.

Three unique features of Zhaozhou Bridge

(1) The "coupon" is smaller than a semicircle. In our country, it is customary to call arc-shaped bridge openings, door openings and other buildings "vouchers". Generally, the coupons for stone bridges are semicircular. But the span of the Zhaozhou Bridge is very large, 37.04 meters from one end to the other. If the bridge is made into a semicircle, the bridge hole will be 1852 meters high. In this way, vehicles, horses and pedestrians crossing the bridge are like crossing a hill, which is very strenuous. The bridge of Zhaozhou Bridge is an arc smaller than a semicircle, which not only reduces the height of the bridge, but also reduces the stone materials and labor required to repair the bridge. It also makes the bridge very beautiful, much like a rainbow in the sky.

(2) "Crash" is empty but not real. The two shoulders of the coupon are called "bump". Generally, the bridges of stone bridges are built with stones, but the Zhaozhou Bridge's bridges are not solidly built. Instead, one or two small arc-shaped coupons are built on each shoulder of the bridge. In this way, four small coupons were added to the bridge body, which saved about 180 cubic meters of stone and reduced the weight of the bridge by about 500 tons. Moreover, when the Mihe River rises, part of the water can flow downward from the small bridge, which not only makes the water flow smooth, but also reduces the impact of floods on the bridge and ensures the safety of the bridge.

(3) Side-by-side cave construction. It uses 28 small coupons arranged side by side to form a large coupon 9.6 meters wide. However, if it is built side by side, the stones in each narrow path are not connected with each other, so it is not as strong as the vertical type. In order to make up for this shortcoming, when the Zhaozhou Bridge was built, iron nails were added between the stones of each narrow pass to connect them as a whole. With narrow bridges built in parallel, even if one is broken, it will not affect the overall situation, it is easy to repair, and the traffic on the bridge will not be affected during bridge repair.

Types and forms of bridges

According to the main materials

Wood, stone, brick, bamboo, rattan, iron, salt, ice bridge

Wooden bridges are the earliest form of bridges. Almost all bridges in my country before the Qin and Han Dynasties were wooden bridges. Such as the earliest single-plank bridges and wooden pillar-beam bridges. Floating bridges appeared around the Shang and Zhou dynasties, and column-arranged wooden beam bridges and outrigger-type wooden beam bridges appeared around the Warring States period.

However, due to the characteristics of wood itself, such as pine, which is perishable and is subject to the strength and length of the material, it is not only difficult to build bridges on wide rivers, but also difficult to build strong and durable bridges. Therefore, the Northern and Southern Dynasties began to Replaced by mixed wood and stone or stone bridges.

Stone bridges and brick bridges. Generally speaking, it refers to a bridge whose deck structure is also made of stone or bricks. However, bridges made of pure bricks are very rare. They are usually constructed of a mixture of bricks, wood, or masonry, while stone bridges are more common. During the Spring and Autumn Period and the Warring States Period, stone pier and wooden beam span-type bridges appeared. In the Western Han Dynasty, they further developed into stone pillar-type stone beam bridges. In the Eastern Han Dynasty, single-span stone arch bridges appeared. In the Sui Dynasty, the world's first open-shoulder single-span bridge was created. Hole curved stone arch bridge, Li Zhaode built a boat-shaped pier porous stone beam bridge in the Tang Dynasty. The Song Dynasty was a period of vigorous development of large-scale stone bridges, creating stone beam bridges spanning several miles across the intersection of rivers and seas, such as Quanzhou's Luoyang Bridge and Ping'an Bridge, as well as large-scale stone bridges such as Beijing's Lugou Bridge and Suzhou's Baodai Bridge. arch bridge.

Bamboo bridge and rattan bridge. Mainly found in the south, especially in the southwest. Generally it is only used on narrow rivers or as temporary crossings. In the early days, it was mainly a kind of cable bridge. In the Southern and Northern Dynasties, the bamboo zipline bridge was called "Zuo Bridge". Later, bamboo cable bridges, bamboo pontoon bridges and bamboo plank bridges appeared. Iron bridges, in ancient times, included iron cable bridges and iron pillar bridges. The former belongs to the cable bridge type, which is more common, and appeared around the Tang Dynasty; the latter belongs to the beam bridge type, and is actually a hybrid bridge of wood and iron, which is very rare, with one example seen in Jiangxi.

Salt Bridge and Ice Bridge. Mainly found in special natural environments. The former is mainly found in the Qinghai Salt Lake area, and the latter is mainly found in the cold northern areas.

According to structure and appearance, there are four basic types: beam bridge, pontoon bridge, cable bridge and arch bridge.

Beam bridge. Also known as flat bridge and empty span bridge, it is a bridge with piers as horizontal distance supports, and then the beams are erected and the bridge deck is flat. This is the most commonly used type of bridge, and it appeared earlier than other bridge types in history. It comes in the form of wood, stone or a mixture of wood and stone. In the pre-Qin Dynasty, beam bridges all used wooden columns as piers. However, this wooden column and beam structure showed its weaknesses very early and could not adapt to the development of the situation. Therefore, they were replaced by stone pillar and wooden beam bridges, such as the multi-span long bridges built in the Qin and Han Dynasties: Wei Bridge, Ba Bridge, etc. Around the time of the Han Dynasty, pile foundation technology was invented, and stone bridge piers appeared, indicating that bridges composed of wood and stone could span wider rivers and withstand the impact of raging floods. However, because the wooden beams on the stone piers were not resistant to wind and rain erosion, bridge houses were built on the bridge to protect the bridge body. This bridge type (covered bridge) was later more common in the south, but it was first seen in the Yellow River Basin. Small and medium-sized stone beams or stone slab bridges are convenient in structure, durable in materials, and labor-saving in maintenance. They are the most popular bridge shape among the people. Especially after the Southern Song Dynasty, they became very popular in Quanzhou, Fujian, and many long stone beams were created. bridge. If a beam bridge has no piers in the middle, it is called a single-span beam bridge; if there is a pier in the water so that the bridge body forms two holes, it is called a double-span beam bridge; if there are more than two piers, it is called a multi-span beam bridge.

Floating bridge. Also known as pontoon bridge, pontoon bridge, and floating truss, it is also called "war bridge" because it is easy to erect and is often used for military purposes. A bridge on the water that is chained and juxtaposed on the water, and the boats are covered with planks for people and horses to pass. If a bridge in the strict sense is marked by a span and piers, then it is not a bridge in the full sense. Floating bridges are mainly built in places where the river surface is too wide, too deep, or where there are large fluctuations, making it difficult for ordinary wooden and stone column beam bridges to help. There are many pillars or iron cows, iron mountains, stone gates, stone lions, etc. on both sides of the pontoon to tie the cables. The Tianjin Bridge built on Luoshui River in Luoyang in the first year of Daye of the Sui Dynasty was the first pontoon bridge to use iron chains to connect ships. Floating bridges are still commonly used in southern my country such as Jiangxi, Zhejiang, Guangxi and other places.

The advantages of pontoon bridges: First, construction is fast. In the second year of Xianfeng in the Qing Dynasty (1852 AD), the Taiping Army besieged Wuchang and built two pontoon bridges across the Yangtze River in just one night. Second, the construction cost is low. Zou Shouyi of the Ming Dynasty once compared stone bridges and floating bridges in "Repairing the Fenglin Floating Bridge": "If you use a stone beam bridge, it will cost a thousand gold, but if you use a floating bridge, it will cost five hundred gold. It can be determined according to needs. "Third, it can be opened and closed at will, and it is easy to dismantle and erect.

The disadvantage is that it has a small load capacity, fluctuates with the waves, and has a weak ability to withstand floods. It often needs to be dismantled in time and needs to be taken care of. The management is cumbersome. The boat, bridge deck and mooring cables need to be repaired and replaced frequently, and the maintenance cost is high. expensive. Therefore, the final destination of many floating bridges develops into wooden beam bridges, stone beam bridges or stone arch bridges.

Rope bridge. Also known as suspension bridge, rope bridge, suspension bridge, etc., it is a bridge suspended with bamboo ropes, rattan ropes, iron ropes, etc. as the backbone. They are mostly built in dangerous valleys with steep banks where strong currents make it difficult to build bridge piers. They are mainly found in the southwest region. The method is to build a house on both sides of the bank, with a rope-tying column and a rope-twisting column in each house, and then use a number of thick ropes to tie them flat, and then lay wooden boards across the ropes, and some also add one or two ropes on both sides. as a handrail. It first appeared in the Qin and Han Dynasties. For example, Qin Li Bing built a Zuo Bridge in the southwest of Yizhou (now Chengdu), Sichuan, also known as "Yili Bridge", which was a bamboo cable bridge. Famous existing ones include the Luding Iron Cable Bridge and the Guanxian Bamboo Cable Bridge built in the Ming and Qing Dynasties. Crossing the cable bridge feels very thrilling, just as the ancients described crossing the cable bridge: "A person is suspended in mid-air, crossing a ravine. If he doesn't stop for a moment, he will fall into a bottomless valley." Zhimeng, a monk in the Tang Dynasty, said: "If you can't see the bottom, you will be shadowed. "In fact, it is actually safe to cross it, just as "Xu Xiake's Travels" commented on the Panjiang Bridge in Guizhou: "It seems to be floating when you look at it, but it is still standing when you practice it."

Arch bridge. It appeared relatively late in the history of bridges in our country, but once the arch bridge structure was adopted, it developed rapidly and became the most vital bridge type among ancient bridges. Even today, it still has broad prospects for continued development. Arch bridges can be divided into stone arches, brick arches and wooden arches. Among them, brick arch bridges are extremely rare and are only occasionally used in temples or gardens. The most common one is the stone arch bridge, which can be divided into single arch, double arch and multiple arch. The number of arches depends on the width of the river. Generally, the arch in the middle should be particularly tall, and the arches on both sides should be slightly smaller. According to the shape of the arch, it can be divided into five sides, semicircle, pointed arch, flat arch, etc. The bridge deck is usually paved with stone slabs and the bridge sides are made of stone railings. The image of the arch bridge was first seen on the portrait bricks of the Eastern Han Dynasty. It was produced by the development of the outrigger wood and stone beam bridge and was influenced by the shapes of tomb arches, water pipes, etc. Documentary records can be found in the "Shui Jing Zhu" of the Southern and Northern Dynasties. The earliest existing object and the most representative one is the Zhaozhou Bridge designed and built by Li Chun in the Sui Dynasty. For the issuance of certificates for stone arch bridges, after the Ming Dynasty, especially in the Qing Dynasty, it was popular to use whole coupons, that is, "barrel issuance".

Other shapes

Flying pavilions and plank roads, channel bridges and towpath bridges, as well as curved bridges, fish pond flying beams and Fengshui bridges.

"Flying Pavilion" is also called Pavilion Road or Fudao, which is an overpass. Cross passages between pavilions in ancient palaces. "Sanfu Huangtu": "On the west side of the palace (referring to the Weiyang Palace of the Han Dynasty), a flying pavilion was built across the city to connect to the Zhang Palace, and a chariot road was constructed to go up and down." The palaces of the Qin and Han Dynasties were connected by pavilion roads, because there were roads up and down. , so it is called Fudao. Qin Shihuang's Pavilion Road leads from Epang Palace to Lishan Mountain, with a pedestrian bridge and a vehicular bridge under it. It can be called the earliest overpass in China. "Plank Road", also known as Zhange, Qiaoge, is a single-arm wooden beam bridge. In steep areas of the mountainous areas, there are wooden roads.

"Channel Bridge" is both a water diversion channel and a bridge for pedestrians. That is to say, a ditch was built on the bridge to divert water. For example, the Huiyuan Bridge in Hongdong County, Shanxi was built in the Jin Dynasty. Therefore, there is still a slang term among Shanxi people: "bridge on water, water on bridge". "Towpath Bridge" is a long ribbon-shaped bridge parallel to the river built to facilitate fiber pulling. It is more common in canal areas in Zhejiang. Some are one to two kilometers or even five or six kilometers long. For example, there is a "hundred-hole Guantang" towpath bridge in Ruanshe, Shaoxing. It was built during the Tongzhi period of the Qing Dynasty. The bridge is more than 380 meters long and has 115 spans. The bridge deck is made of three strips. It is made of stones, with a flat bottom connected to the water.

"Curved bridge" is a unique bridge style in gardens, so it is also called garden bridge. Bridges, paths, and corridors are all passages for visitors to enjoy the scenery in the garden. "Scenery is better than curves." Therefore, bridges in gardens are often made with folded corners, such as the Jiuqu Bridge, to form a folded line that swings back and forth, looking left and right, to achieve the effect of extending the scenery line and expanding the landscape picture. Curved bridges are generally composed of stone slabs and railings. The stone slabs are slightly higher than the water surface and the railings are low, making the bridge seem to be separated from the water surface and the space seems to be separated but not separated. It has a particularly implicit and endless meaning.