| A bridge is an artificial structure to span a river, ravine, lake or other physical obstacle. A bridge built over a road is called an over bridge and a bridge built on a ravine or gorge, a viaduct.
Design:
As a rule, bridges consist of spans and supports. The spans serve for the perception of loads and transfer them to the supports; Roadways, pedestrian crossings and pipelines can be positioned on the spans. Supports transfer loads from the spans onto the base of the bridge.
The spans consist of load bearing designs: beams, frames, diaphragms (cross-sectional beams) and proper roadway plates. The static layout of spans can be arch, braced, framed, cable-stayed or combined; It determines the bridge type according to design. Usually, the spans are rectilinear, however, if required (for example, while carrying out construction of flyovers and junctions), they are given complex shapes: spiral, circular etc.
The shapes of supports can vary a lot. Intermediate supports are called breakers and coastal — abutments. The abutments serve for connecting the bridge with approach embankments.
Metal (steel and aluminum alloys), reinforced concrete, concrete, natural stones and wood serve as materials for constructing bridges.
Construction of bridges:
The first stage in construction of bridge (and most expensive — up to 50 % of the expenditure from the total construction cost) is the erection of supports. The support is erected in open foundation pits or by sinking piles, sunken shafts, caissons and prefabricated shell structures in the soil. Piles (predominantly are made of reinforced concrete) are used mainly while constructing small and medium bridges. They are sunk inside the soil by making use of diesel hammers and electric silent pile drivers. The shell structures with a diameter of up to 3m are mainly used while erecting big bridges.
The spans are usually established on supports using erecting cranes. While constructing big bridges, spans are often prefabricated on the coast and they are moved (pulled) along the supports from one coast onto the other. The cantilevered method of installation assumes the rise in structure from the support of the bridge in its bay. In this case, the use of cantilevered erection with the help of a crane, moving along the already constructed part (for metallic spans) or the balanced erection by manufacturing separate elements at a factory and their subsequent transportation to the site (for reinforced concrete).
Construction of transporter bridges takes place differently: it begins with the installation of pillars; then temporary cables are suspended on them. The winding of the main cables of the bridge is done with the help of temporary cables and after which, suspensions and a stiffening beam is mounted.
Classification:
According to field of application
According to field of application, the bridges are divided into:
Railway bridges;
Road bridges;
Bridges for underground railways;
Pedestrian;
Combined (for example, rail – road).
Pipelines, aqueducts (bridges, used for water transportation) and viaducts (bridges through ravines or gorges; connect points at equal height) also exist.
According to design:
According to design, the bridges are divided into:
Beam bridges — the simplest type of bridges. These bridges are intended for covering small spans. Spans — beams, covering the distance between supports.
Cantilever bridge — consists of cantilevers. Presently, this type of bridge design is very rarely used;
Suspension bridges — a bridge in which the main load-bearing design is done from flexible elements (cables, ropes, chains, etc.), working on tension and a roadway is suspended. This type presents the largest spans of bridges with respect to length and height;
Cable-stayed bridge — a variant of suspension bridges: a cable-stayed frame, made from rectilinear steel ropes, accomplishes the role of the main load-bearing design. The big Obukhovsky Bridge in St.Petersburg can serve as an example;
Arch bridge — the main load-bearing designs are arches or copes. In this case, not only vertical but also horizontal stresses are transferred onto supports;
Pontoon or floating bridge — is a temporary bridge on floating supports.
It is necessary to separately note hump-back bridges, which are distinguished by their shape: they are essentially cambered upwards. |
