Bridges with curved girders as the main load-bearing members. The main beam can be a solid web beam or a truss beam (vierendeel beam). Solid web beam is simple in shape, easy to manufacture, install and maintain, and is widely used in medium and small span bridges. However, solid web beam is not economical in material utilization. In the truss beam, each member of the truss basically only bears axial force, which can make good use of the strength of the member material. However, truss beams are complex in structure and labor-consuming, and are mostly used for long-span bridges. Truss beams are generally made of steel, but prestressed concrete or reinforced concrete can also be used, but it is rarely used. Truss beams used to be made of wood, but now they are rarely used because of their poor durability. Reinforced concrete and prestressed concrete are mainly used for solid web beams, and steel plate beams or steel box beams can also be used. The earliest forms of solid web beam bridges are wood beam bridges made of logs and stone slab bridges made of stones. Due to the size, performance and resources of natural materials, wooden bridges are basically not used now, and stone slab bridges are only used as small-span pedestrian bridges.
Second, the arch bridge is a bridge with the arch frame (called the main arch ring) bearing axial pressure as the main load-bearing component.
1. According to the static diagram of the main arch ring, the arch car can be divided into three-hinged arch, two-hinged arch and non-hinged arch (Figure 3 is the schematic diagram of arch bridge form).
(1). The three-hinged arch is statically indeterminate, and its overall stiffness is low. Especially, the deflection curve produces an angle at the hinge of the vault, which makes the impact of live load on the bridge stronger, which is not conducive to driving. The structure and maintenance of vault hinge are also very complicated. Therefore, the three-hinged arch is generally not used as the main arch ring except for the belly arch ring sometimes used for buildings above the arch.
(2) The two-hinged arch cancels the vault hinge, so the structure is simpler than that of the three-hinged arch, the overall stiffness of the structure is better than that of the three-hinged arch, and the maintenance is easier than that of the three-hinged arch, and the additional internal force caused by the settlement of the bearing is smaller than that of the non-hinged arch. Therefore, the double-hinged arch bridge can be used where the foundation conditions are poor and it is not suitable to build a hinged arch.
(3) The hinged arch is a cubic statically indeterminate structure. Although the additional internal force caused by bearing settlement is large, the internal force distribution of the arch under load is relatively uniform, the structural stiffness is large, the structure is simple and the construction is convenient. Therefore, hinged arch is a widely used form of arch bridge, especially masonry arch bridge and reinforced concrete arch bridge.
2. According to the form of main arch ring, arch can be divided into plate arch, rib arch, hyperbolic arch, box arch and truss arch. (Figure 4 is a schematic diagram of the main arch ring).
① Plate arch: The arch ring is a rectangular solid section with good lateral integrity, small section height and simple structure, but poor bending moment resistance, which is generally used for masonry arch bridges. Jiuxigou Bridge in Sichuan, built in 1972, is a stone arch bridge with a span of 1 16m, and it is the largest stone arch bridge in the world at present.
(2) rib arch: the arch ring is composed of two or more arch ribs, which are connected by beams. The shape of the arch rib can be rectangular, I-shaped, box-shaped or circular. Its bending performance is better than that of plate arch, and it uses less materials, but its manufacture is more complicated than that of plate arch, and it is mostly used in reinforced concrete arch bridges or steel arch bridges. 1960 The Enskovyo Bridge in Sweden, with a span of 278 meters, is the largest steel tube arch bridge at present.
③ Hyperbolic arch: an arch bridge adopted in China after 1960s. In addition to the arch rib, it also has a small arch composed of arch wave and arch plate, which connects the whole arch ring into a whole. During construction, arch ribs and arch waves can be prefabricated, and arch slabs can be poured after installation, so as to reduce the lifting weight, which can provide a new structural form and a simple and easy construction method for concrete arch bridges without using arch frames or simply using brackets. But measures should be taken to ensure the integrity of the arch ring. He Qian Bridge, built in 1969 and with a span of150m, is the largest hyperbolic arch bridge at present.
④ Box arch: The cross section can be an integral multi-chamber box or a split box. Concrete or reinforced concrete box arches can also be built without support. Its integrity, lateral stability and torsional performance are better than those of hyperbolic arch structure, but it is not as simple and economical as hyperbolic arch in medium and small span. Clark Bridge in Yugoslavia, 1979 completed, with a span of 390 meters, is the largest reinforced concrete box arch bridge in the world.
⑤ Truss arch: The arch ring is composed of trusses, which can be made into truss rib arch or shoulder arch (Figure 5 is a schematic diagram of truss arch). The material consumption of truss arch is economical, but some members of truss will bear tension, so it is mainly used for steel arch bridges or prestressed concrete arch bridges. The American New River Bridge, completed at 1976 with a span of 5 18m, is the largest steel truss arch bridge at present.
The shape of the main arch ring of the arch bridge along the span direction can be made into an equal-section arch with the same cross-section size along the arch axis, or a variable-section arch with the cross-section size gradually changing from the arch foot to the vault. Variable cross-section arch can better adapt to the change of internal force of arch ring, and the material is economical; Equal section arch is widely used because of its simple structure and convenient construction.
The arch axis shape of the main arch ring will directly affect the stress of the arch ring section. Generally, try to make the arch axis coincide with the arch ring pressure line under load to reduce the section bending moment. When the elastic compression of arch ring is ignored, the pressure line of arch under uniform load is parabolic; According to the shape of the arch axis, the pressure line of the arch will be catenary under the action of the distributed load that increases gradually from the vault to the arch foot; The arc line is the simplest, which is beneficial to construction. Therefore, these lines have become the arch axis shapes commonly used in arch bridges.
3. According to the different architectural forms on the arch, the arch can be divided into solid arch and hollow arch. Solid web arch is to fill all the space between the main arch ring and the bridge deck with filler, which is generally used for small-span bridges; On the other hand, the vierendeel arch is provided with a web hole that runs transversely through the main arch ring, which is generally used for bridges with medium span or above. The earliest existing hollow arch bridge in Zhao Zhouqiao.
Under vertical load, arch rib as a load-bearing structure mainly bears pressure. The support of arch bridge should bear not only vertical force but also horizontal force. Therefore, the requirements of arch bridge for foundation and foundation are higher than those of beam bridge. The following pictures show a through arch bridge (the deck is above the arch rib), a half-through arch bridge (the deck is partly above the arch rib and partly below the arch rib) and a through arch bridge (the deck is below the arch rib). An arch bridge for people, words and deeds can directly lay the deck on the arch ribs. However, the bridge deck of an arch bridge passing through modern means of transportation must maintain a certain straightness and cannot be directly laid on the curved arch rib, so the bridge deck should be indirectly supported on the arch rib through columns or suspenders.
3. Cable-stayed bridge: It is composed of the main girder, the steel cables of the skew main girder and the pylon supporting the steel cables (Figure 9 is the schematic diagram of the cable-stayed bridge). The cables of the cable-stayed bridge are stretched into a straight line, and the overall structure geometry remains unchanged, and the stiffness is greater than that of the suspension bridge. The main beam is similar to a continuous beam on an elastic support. The span of a cable-stayed bridge is generally between a girder bridge and a suspension bridge. Brudona Bridge, 1977, which was built in France and spans 320 meters, is the largest prestressed concrete cable-stayed bridge in the world. 1975 French-built Loire River steel cable-stayed bridge with a main span of 404 meters. Cable-stayed bridges have single tower or twin towers in structure, and cables are arranged on one side or two sides, with dense cables or few cables. The arrangement of cables also has different radiation forms, and there are also many types of hinged or fixed connections among towers, beams and piers.
Cable-stayed bridge is called cable-stayed bridge in Japanese, cable-stayed bridge in German and cable-stayed bridge in English. Cable-stayed bridge is formed by pulling the beam on the tower with several cables. Compared with perforated beam bridges, stay cables are (elastic) fulcrums, not piers, thus increasing the span of the bridge.
This type of cable-stayed bridge has existed since ancient times. However, the force on the stay cable is difficult to calculate and control, so it has not been developed and widely used. Until the middle of this century, due to the appearance of electronic computer, the problem of cable force calculation was solved, the adjustment device was perfected, and the problem of cable force control was solved, making cable-stayed bridge the fastest developing and most widely used bridge type in recent 50 years.
4. Suspension bridge, also known as suspension bridge, is a bridge with cables or chain cables bearing tension as the main load-bearing components. A suspension bridge consists of suspension cables, pylons, anchorages, suspenders and deck system (Figure 6 is a schematic diagram of the suspension bridge). 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 strand, 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 material saving and light weight, the span of the suspension bridge is the largest among all kinds of bridges, and the span can reach more than 1000 meters. The Humbil Suspension Bridge in England was built at 198 1 with a span of 14 10 meter, which is the largest bridge in the world at present. The main disadvantage of suspension bridge is its low stiffness, and it is easy to produce large deflection and vibration under load, so corresponding measures should be taken.
According to the stiffness of bridge deck system, suspension bridges can be divided into flexible suspension bridges and rigid suspension bridges. Generally, the deck system of a flexible suspension bridge is not provided with stiffening beams, so the stiffness is relatively small. Under the vehicle load, the bridge deck will be S-shaped with the change of the shape of the suspension cable, which is not conducive to 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, which has great stiffness. Stiffened beams can bear vertical loads together with the whole bridge structure. In addition to the above forms, in order to enhance the stiffness of suspension bridges, double-chain suspension bridges and suspension bridges with inclined suspenders can also be used, but their structures are more complicated.
A bridge whose deck is supported by suspended cables (usually called large cables) is called a suspension bridge. Suspension bridge means "suspended bridge" in English, so it is also translated as "suspension bridge". In most cases, the suspension system of "suspension bridge" is composed of "cable", so it is translated as "suspension bridge". However, in some cases, "cable" is also composed of rigid rods or key rods, so it cannot be translated into "suspension bridge". Contrary to the arch rib, the suspension cable section only bears tension. Simple suspension bridges used only by people and animals often lay the deck directly on suspension cables. Suspension bridges with modern means of transportation are not. In order to keep the bridge deck straight, the bridge deck is suspended from suspension cables with slings. Different from arch bridges, arch ribs as load-bearing structures are rigid, while suspension cables as load-bearing structures are flexible. In order to avoid the deformation of bridge deck and suspension cables when vehicles pass, modern suspension bridges are generally equipped with rigid beams (also known as stiffening beams). The bridge deck is laid on a rigid beam, and the rigid beam is suspended on a suspension cable. The suspension cables of modern suspension bridges are generally supported on two towers. Saddle-shaped brackets are arranged at the top of the tower to support suspension cables. The end of the suspension cable under great tension is fixed in the foundation through anchorage, and part of it is fixed at the end of the rigid beam, which is called self-anchored suspension bridge.