If you hold both ends of a stick with both hands and press the middle part with your knees, the force on the stick is similar to that of a simply supported beam. It's just that the simply supported beam is stressed up and down, and the stick is stressed back and forth. That is, while being under tension, it is also under pressure. Simply supported beam is the simplest bridge structure, which is called statically indeterminate structure in theory, that is, one beam is placed on two columns. The connection between beam and column is a simple mechanical connection, and the stress has nothing to do with the height of column. Simply supported beam is simple, but it is not suitable for long-span bridges. Because in the case of long span, the weight of simply supported beam itself may exceed the weight of load, which is economically unreasonable.
It is obviously different from a continuous beam, which has three or more columns under a beam. The height change of each column, such as uneven settlement, affects the stress distribution of all columns, which is called statically indeterminate structure or statically indeterminate structure. Other bridge types include arch bridge, T-shaped structure, suspension bridge, cable-stayed bridge and truss.
Arch bridge is a bridge with arch structure as the main stress component, which is characterized by main stress, and is especially suitable for occasions without tensile materials (mainly steel). Ancient long-span bridges were basically arch bridges, because the building materials at that time were all stone or wood. And wood is obviously not suitable for long-term corrosion resistance and large-scale stress. After the founding of the People's Republic of China and before the reform and opening up, a large number of concrete bridges were built in China, which is also because of this reason (lack of steel).
T-shaped structure, referred to as T-shaped structure for short, refers to cantilever structure extending from pier to both banks. After being connected to two banks or another T-shaped structure, the connecting part is stressed like a hinge. The force is more reasonable. Because of the different cross sections of bridges, they are generally used for large bridges, so the cast-in-place concrete method is used for construction.
Suspension bridges are located on both sides of rivers or at the height of piers, and mooring cables (so-called ropes, that is, soft ones) are the main stress structures. Because it is soft, the suspension bridge has poor rigidity, which is also commonly known as rickety. Hemp rope was used before, and steel wire rope was also used after the industrial revolution. Suitable for long-span bridges.
Cable-stayed bridge means that many steel cables radiate from a high tower to every section of the beam. Originally, the section of the beam was in tension at the upper part and in compression at the lower part, but the cable changed the stress state and gave full play to the material's ability. Generally speaking, cable-stayed bridge is the most reasonable bridge type.
Truss is also a widely used bridge form, and many steel bridges use trusses, such as the Yangtze River Bridge in Nanjing and Wuhan, which are generally continuous beams composed of steel trusses and steel arches. Of course, there are some concrete truss arch bridges.
The advantage of simply supported beam is its simple structure, and it can even be produced in factories. Just like standard parts such as screws, the length, width and bearing capacity of beams are a series of fixed values, which designers can choose from the product list. There is a bridge factory near Fanjiacun, Beijing, which produces reinforced concrete beams. Generally, simply supported beams are used in bridge sites with good geological conditions and low pier cost, and long bridges can be built with more spans. However, in the bridge site with complex geological conditions, there are few places where piers can be set up, or the pier cost is high, such as the railway viaduct across the valley, so only long-span bridges can be built.
Large and medium-sized bridges usually use cast-in-place beams. Bridges are all built on the construction site because of inconvenient transportation. The method is to bind the steel skeleton first, and then pour concrete. The previous technology was to weld the steel bars of the next section in one section of concrete. After the early strength of concrete is formed, the joint surface is chiseled away, and then new concrete is poured. Later, it was changed to the method of template sliding. Under the package of the formwork, there are several sections of concrete, which are solidified, semi-solidified and newly poured. When the formwork moves forward alternately, continuous construction can be carried out.
Strictly speaking, cantilever beam is a kind of cast-in-place beam. The above construction techniques include ground casting and pier casting. Some medium-sized bridges are poured on the ground of the site, and then hoisted to the piers by cranes or bridge erecting machines. The bridge, because there is no such a big bridge erecting machine, is poured on the pier. The difference between them lies not only in the location of the site, but also in the fact that on the ground, after the strength of cement is formed, it is hoisted and then begins to bear gravity as a beam. On the ground, it is flat. Before the strength of cement is fully formed, the cantilever beam will bear the gravity of the beam, so it should be considered in the design and calculated separately.
Therefore, the main problem is the supply of concrete. Whether it is on-site mixing or special vehicles, it is difficult to provide concrete for all large bridges at one time.