Suspension bridge

Suspension bridge

This entry is compiled and applied by "popular science China" Science Encyclopedia

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Suspension bridge, also known as suspension bridge, refers to a bridge with cables (or steel chains) suspended and anchored on both sides of the bridge (or both ends of the bridge) as the main load-bearing components of the superstructure. The geometric shape of the cable is determined by the equilibrium condition of the force and is generally close to a parabola. Many suspenders are hung down from cables to hang the bridge deck. Stiffening beams are often set between the bridge deck and suspenders to form a composite system with cables to reduce the deflection caused by loads.

principle

The biggest force in suspension bridge is the tension in suspension cable and the pressure in tower. Because the tower is basically free from lateral force, its structure can be made quite slender. In addition, the suspension cable has a certain stabilizing effect on the tower. If the weight of the suspension cable is ignored in the calculation, then the suspension cable forms a parabola. In this way, the calculation process of suspension bridge becomes very simple. The suspension cables of old suspension bridges are usually iron chains or iron bars connected together. Modern suspension cables are usually multi strand high strength steel wires.

structure

The structure of suspension bridge was invented in the early 19th century, and many bridges use this structure. Modern suspension bridge is evolved from cable bridge. The scope of application is mainly for long-span and super long-span highway bridges, which are widely used in today's large-span bridges. It is the main form of long-span bridge.

Suspension bridge is a kind of bridge whose main load-bearing components are cables or chain cables bearing tensile force. It is composed of suspension cables, pylons, anchorages, suspenders, deck systems and other parts. The main load-bearing component of suspension bridge is suspension cable, which mainly bears tensile force and is generally made of steel (steel wire, steel cable, etc.) with high tensile strength. Because the suspension bridge can make full use of the strength of materials, and has the characteristics of saving materials and light weight, the span capacity of suspension bridge is the largest in all kinds of bridges, and the span can reach more than 1000 meters. The Akashi Strait Bridge built in 1998 has a span of 1991 meters, which is the largest bridge in the world. The main disadvantage of suspension bridge is its small stiffness, and it is easy to produce large deflection and vibration under load, so corresponding measures should be taken.

performance

According to the stiffness of bridge deck system, suspension bridge can be divided into flexible suspension bridge and rigid suspension bridge. Generally, the deck system of flexible suspension bridge is not equipped with stiffening beam, so its stiffness is small. Under the action of vehicle load, the deck will produce S-shaped deformation with the change of suspension shape, which is unfavorable to the driving. But its structure is simple, and it is generally used as a temporary bridge. The deck of rigid suspension bridge is strengthened by stiffening beam, so the rigidity is large. The stiffening beam can bear the vertical load with the whole structure of the bridge. In addition to the above forms, in order to enhance the stiffness of suspension bridge, double chain suspension bridge and inclined suspender suspension bridge can also be used, but the structure is more complex.

The bridge whose deck is supported on suspension cable (usually called cable) is called suspension bridge. It means "suspended bridge", so it is also translated as "suspension bridge". The suspension system of "suspension bridge" is mostly made of "cable", so it is translated as "suspension bridge", but in some cases, "cable" is also made of rigid rod or key rod, so the translation of "suspension bridge" can not cover this kind of bridge. Contrary to arch rib, the section of suspension cable only bears tension. The simple suspension bridge is only used for people and animals to walk, and the deck is often directly paved on the suspension cable. In order to maintain the flatness of the bridge deck, it is necessary to hang the bridge deck on the suspension cable with sling. Different from the rigid arch rib used as the load-bearing structure of arch bridge, the flexible suspension cable is used as the load-bearing structure. In order to avoid the deformation of the bridge deck along with the suspension cables when the vehicles drive by, modern suspension bridges are generally equipped with rigid beams (also known as stiffening beams). The bridge deck is paved on the rigid beam, and the rigid beam is suspended on the suspension cable. The suspension cables of modern suspension bridges are generally supported on two pylons. The top of the tower is provided with a saddle bearing for supporting the suspension cable. A self anchored suspension bridge is one in which the end of a suspension cable bearing a large tensile force is fixed in the foundation by anchorage, and some are fixed in the end of a rigid beam.

characteristic

Compared with other bridge structures, suspension bridges can use less material to span longer distances. Suspension bridges can be built high enough to allow ships to pass underneath. There is no need to build temporary piers in the center of the bridge when building the bridge. Therefore, suspension bridges can be built in deeper or faster water flow.

The suspension bridge is more flexible, so it is suitable for the needs of gale and earthquake areas. The more stable bridge must be more solid and heavy in these areas.

The suspension bridge is not strong enough, so the traffic must be temporarily interrupted in strong wind.

Suspension bridge should not be used as heavy railway bridge.

The tower of suspension bridge exerts a great force on the ground, so if the ground itself is soft, the foundation of the tower must be very large and expensive.

The suspension cable of suspension bridge is not easy to replace after corrosion.

Construction method

If the tower is to be built on the water, caissons should be used to squeeze out the soft stratum to build a fixed foundation where the tower is to stand. If the rock layer below is too deep to be reached by caisson, it is necessary to use deep drilling to reach the rock layer or build a very large artificial concrete foundation. This foundation has to extend out of the water. If the tower is to be built on land, its foundation must be very deep, with concrete, boulders and steel structures on the foundation to build piers. Some bridge piers are part of the bridge deck. In this case, the height of the piers should at least reach the height of the bridge deck.

At the top of the tower is a smooth structure called saddle. The saddle may have to be fixed after the bridge is completed. The anchor is fixed in the rock, along the path of the future suspension fiber up a temporary rope or a group of lines. Another strand is suspended above the first strand, on which a pulley can run. This pulley can move from one end of the anchorage to the other end of the anchorage. Each suspension cable needs one such pulley. One section of a high-strength steel wire with a general diameter less than 1 cm is fixed in an anchor, and the other end is fixed on the pulley and thus pulled to the anchor at the other end, and then fixed on the anchor, and then the pulley returns to the anchor from which it starts to pull the next high-strength steel wire or from the direction it is in Next strand of high strength steel wire.

After the steel wire is pulled, antirust treatment should be carried out, so that multiple strands of high-strength steel wire are pulled to connect the anchorages at both ends. Generally, the cross-section of these wires is hexagonal. They are temporarily tied together. After all the wires are pulled, they are squeezed together by a high-pressure hydraulic mechanism and other wires. In this way, the cross-section of the suspension cable is circular.

The cable clamp should be added at the position of equal distance on the suspension cable, and the suspension cable with the length calculated in advance should be supported on the cable clamp. The other end of these suspension cables will fix the bridge deck in the future, using special cranes, and the bridge deck will be hung on the suspension cables one by one. The crane can be hung on its own suspension cable or on a special temporary cable. The bridge deck can be lifted from the boat under the bridge or transported from both ends of the bridge to where they should be. When all bridge decks are suspended, the bridge deck can reach the planned curve by adjusting the suspension cables. Generally, the deck of a bridge on water is arched to facilitate the passage of ships under the bridge. The deck of a suspension bridge on land is usually flat. After the completion of the bridge deck, other details can be carried out, such as drainage and waterproof system, expansion joint, lighting, railings, painting, paving and so on.

history

Suspension bridge is one of the main forms of super long span bridges. Except for Sutong Bridge and Hong Kong stonecutters bridge, other cable-stayed bridges with spans over 1000m are suspension bridges. If carbon fiber with light weight and high strength is used as main cable, its ultimate span can exceed 8000m in theory.

The history of suspension bridge is ancient. Early tropical primitive people used vines, bamboos and tree stems in the forest to make suspension bridges to cross streams. The suspension cables used were vertical, cable-stayed or a mixture of the two. The primitive rattan bamboo bridges in Borneo, Laos and Java are all the rudiments of early suspension bridges. However, the earliest form of suspension bridge with written records belongs to China, and up to now, it still affects the development of suspension bridge form in the world.

As far back as the third century BC, "Zuo" (bamboo cable bridge) was built in Sichuan, China. In Yanyuan County annals of Sichuan Province, it is recorded that "in the 30th year of King Nan of Zhou Dynasty (285 BC), Qin stayed in Shu and took Zuo as his first name. Li Bing (256-251 BC) built seven bridges. One of the seven bridges is Zhusuo bridge. It can be seen that at least in the third century BC, China has recorded the bamboo cable bridge.

As early as 50 BC (the 4th year of Ganlu, Emperor Xuandi of the Han Dynasty), a 100 meter long iron cable bridge was built in Sichuan. In 1665, Xu Xiake had a travel note entitled "the story of the iron cable bridge", which was translated to the west by missionary martini. The book recorded in detail a 122m span iron cable bridge in Guizhou in 1629. In 1667, after the French missionary Kircher returned from China, he wrote a book named "China's miracle", in which he recorded the LanJin iron cable bridge in Yunnan in 65 ad. The book has been translated into many languages and translated into English

Chinese PinYin : Xuan Suo Qiao