Write down three bridge structures you know. What are their advantages?

According to the main load-bearing structure systems, there are beam bridges, arch bridges, suspension bridges, rigid frame bridges, skew Zhang Qiao bridges and composite system bridges (Figure 4). The first three are the basic systems of the bridge.

According to the building materials of bridge superstructure, there are wooden bridge, stone bridge, concrete bridge, reinforced concrete bridge, prestressed concrete bridge (sometimes referred to as concrete bridge), steel bridge and composite beam bridge. Wooden bridges are easy to corrode and are often used as temporary bridges. Stone and concrete have high compressive strength and low tensile strength, and are mainly used for arch bridges. Reinforced concrete bridge is a combination of compressive concrete and steel bars, which has good tensile and compressive properties and is mainly used for beam bridges and arch bridges with small spans. Prestressed concrete bridges are made of high-strength steel bars (wires) and high-grade concrete, which can reach a much larger span than reinforced concrete and the available structural system is much wider than reinforced concrete bridges. Steel bridges are made of structural steel, and are now commonly used in solid web girder bridges and long-span truss girder bridges, arch bridges, skew Zhang Qiao bridges and suspension bridges. Its main advantages are fast construction speed and large spanning capacity; Disadvantages are large steel consumption and high maintenance cost. Composite beam bridge, also known as composite beam bridge, is a bridge composed of two different building materials, usually referring to a bridge composed of steel beams and reinforced concrete decks, which can save steel.

In addition, there are bridges built with lightweight concrete, aluminum alloy, glass fiber reinforced plastic and other building materials.

According to the purpose, there are highway bridges, railway bridges, highway-railway bridges and city bridges. The live load of highway bridges is generally smaller than that of railway bridges, but the action point of live load (wheels) changes in the transverse direction of the bridge, the bridge deck is wider and the allowable deflection of the bridge is larger. The live load of railway bridge runs along the track, and its lateral position on the bridge remains unchanged. The deck system is easy to arrange, but the deck is usually narrow. In the design of long-span railway bridge, it is necessary to widen the bridge due to lateral stability, stiffness and wind vibration. Because of the large live load and small allowable deflection, the degree of freedom in selecting structural system is smaller than that of highway bridges. Bridges used for roads and railways on the same bridge site are called highway-railway bridges. Roads and railways are generally arranged on the upper and lower planes respectively; It can also be arranged on the same plane, and the roads are arranged on both sides of the railway, but the running performance is poor. The urban bridge structure is close to the highway bridge, but the carriageway and sidewalk are wider and the height of the bridge is lower, so as to reduce the length of the approach road at the bridgehead and the amount of fill; When passing through mixed traffic, the longitudinal slope of the bridge should not be greater than 2%; In the design, utilities pipes (telecommunications, lighting, running water, heating and rainwater pipes, etc.) are installed. Considering crossing the bridge, it will not hinder the repair and maintenance of the bridge, but it is not allowed to lay high-voltage transmission lines, gas pipes, oil pipes and sewage pipes on the bridge. Urban bridges should be regarded as one of the important artistic buildings. It should be noted that the shape of the bridge itself should be coordinated with the surrounding landscape, and the artistic requirements for the bridge head, railings and lamp posts are also higher. People in the upper reaches of the city bridge are willing to stop, so it is best to choose a deck bridge that does not affect the scenery.

In addition, there are footbridges, airport bridges, canal bridges, water supply bridges (crossing bridges) and pipeline bridges for oil supply, gas supply and pulverized coal supply.

According to the obstacles, there are river-crossing bridges, valley-crossing bridges, overpass bridges and overhead line bridges. The length and height of the river-crossing bridge should meet the requirements of flood discharge and navigation. The bridge on the main river channel is called the long-span main bridge. The rest are called approach bridges, and their span is generally determined by economic conditions, so standard design should be given priority (see standard design of bridges). The length and height of the bridge across the valley are determined by the topography. When the overpass bridge is a three-dimensional crossing line (highway, railway, etc. ), one line crosses another line, also called overpass; For example, underground crossing existing lines is called underpass bridge. Overhead line bridge is a dry bridge built on the ground or above the road for vehicles to travel. It replaces the overhead line of embankment with bridge structure, which can avoid line crossing and improve transportation capacity.

According to the position of the bridge deck, there are deck bridges, half-through bridges, deck bridges and double-deck bridges. The deck placed on the main load-bearing structure is called a through bridge (Figures 4a and b), and the deck near the lower edge of the main load-bearing structure is called a through bridge (Figure 3). The bridge between the upper and lower edges is called a half-through bridge; A double-deck bridge with decks on both sides is called a double-deck bridge. Through bridge has the advantages of simple structure, easy maintenance, convenient manufacture and erection, saving the number of pier masonry and wide field of vision, and is often preferred in bridge design. Half-through bridge beams and half-through bridge beams have the advantages of small bridge construction height, so they are used according to design requirements. Double-deck bridges are mainly used for highways and railway bridges.

According to the shape of the bridge plane, there are orthogonal bridges, oblique bridges and curved bridges. The center line of the orthogonal bridge is orthogonal to the flow direction of the main river (or the center line of the intersection line). The center line of the skew bridge intersects with the flow direction of the main river. The skew degree is generally expressed by the normal angle between the center line of the bridge and the supporting line. The skew bridge is complex in stress and structure, and there are many materials. Curved bridge is a bridge whose main load-bearing structure axis is arranged along the curve, and its stress and structure are also complicated. In order to facilitate driving, the bridge deck should be widened according to the requirements of the line.

According to the production method, concrete bridges are divided into cast-in-place bridges and assembled bridges. The latter component is prefabricated in the factory (site) and transported to the construction site for assembly and installation. Its advantages are industrialization and mechanization of bridge manufacturing, cost reduction, speed improvement and quality assurance. There is also a combination of assembly and cast-in-place concrete bridges. Steel bridges are generally fabricated.

According to the bridge length, bridges need to be classified according to the bridge length in terms of technical requirements and maintenance facilities.

According to the service life, it is divided into temporary bridges, permanent bridges and semi-permanent bridges. Temporary bridges are simple in structure and can only be used in a limited short time, or used to maintain traffic until the permanent bridge is completed. The permanent bridge is a long-term bridge, which needs to be designed according to the specified design flood frequency, deck width and inspection and maintenance equipment. In the general design of semi-permanent bridges, the substructure is a permanent bridge, while the superstructure is temporary.

Other special bridges include movable bridges, military bridges and flood control bridges. Movable bridge refers to a bridge whose span structure can be moved or rotated to expand or open the free passage under the bridge, which is mostly used in the lower reaches of the river near the sea port. Military bridge is a temporary bridge for military purposes, which is used to cross obstacles such as rivers, canyons, trenches and craters. It is usually required that the materials are light in weight, high in strength, interchangeable in components, simple in disassembly and assembly, and can be quickly assembled into bridges with various spans and different bearing capacities. Shuiman Bridge is a semi-permanent bridge built when the traffic of Grade III and IV highways is restricted. The bridge deck is built below the design flood level elevation, and the flood overflows in flood season, so masonry materials are often used to build it.

Bridges-basic characteristics

Beam bridges include simply supported plate girder bridges, cantilever girder bridges and continuous girder bridges, among which the span capacity of simply supported plate girder bridges is the smallest, with a span of 8-20m in general. The maximum span of continuous beam bridge in China is below 200m, and it has reached 240m abroad.

Under the vertical load, the supports at both ends of the arch bridge produce vertical reaction and horizontal thrust. It is the horizontal thrust that greatly reduces the mid-span bending moment and increases the span capacity. Theoretically, the ultimate span of concrete arch is about 500m, and the steel arch can reach1200 m. It is this thrust that requires good geological conditions when building arch bridges.

Rigid frame bridges include T-shaped rigid frame bridges and continuous rigid frame bridges. The main disadvantage of T-shaped rigid frame bridge is that there are many expansion joints on the deck, which is not conducive to high-speed driving. The continuous rigid frame beam is continuous and seamless, and the driving is stable. There is no system conversion during construction. The largest span in China has reached 270m (auxiliary channel bridge of Humen Bridge).

Cable-loaded bridges (cable-stayed bridges and suspension bridges) are the best design for building bridges with long spans. The bridge deck of a highway or railway is suspended in the air by cables, and cables are suspended between bridge towers. The main span of cable-stayed bridge can reach 890m, and that of suspension bridge can reach1991m.

Composite system bridge beam includes beam-arch composite system, such as tied arch, truss arch and multi-span arch beam structure, and beam-frame composite system, such as T-shaped rigid frame bridge.

Truss beam bridge: It has a strong beam with brackets at both ends. The earliest bridges were built according to this idea. They are just tree trunks or stones across the river. Modern truss girder bridges usually use long hollow trusses made of steel or concrete as beams. This makes the bridge light and strong. Bridges built in this way are called box girder bridges.

Cantilever bridge: The bridge body is divided into several long and solid parts, similar to truss bridge, but each part is supported by the middle instead of the two ends.

Arch bridge: The arch bridge pushes the ground at both ends of the bridge and bears the stress of the main span. Modern arch bridges usually adopt light open structure.

Suspension Bridge: Building a bridge with a very long span is the best design. The bridge deck of highway or railway is suspended in the air by steel cables, which are firmly suspended between bridge towers. Some older suspension bridges use chains, and some even use ropes instead of steel cables.

Cable-stayed bridge: Cable is tied to the bridge column. The steel cable supports the weight of the bridge deck and transfers the weight to the bridge column, which puts great pressure on the bridge column.

Glass bridge: a bridge made of pure glass.

Covered Bridge: A bridge with pavilions, called Pavilion Bridge or Covered Bridge, can provide shade for tourists and increase the shape change of the bridge.