As a rule, bridges consist of spans and piers. Span structures are used to absorb loads and transmit them to the supports; they can carry the roadway, crosswalk, and pipeline. The supports transfer the loads from the superstructures to the foundation of the bridge.
Span structures consist of load-bearing structures: beams, trusses, diaphragms (cross beams) and the actual slab of the roadway. The structural arrangement of the span structures can be arch, beam, frame, cable-stayed or combined; it determines the type of bridge by design. Usually the spans are straight, but if necessary (e.g., when building overpasses and road junctions) they are given a complex shape: spiral, circular, etc.
Span structures are supported by supports, each consisting of a foundation and a supporting part. The forms of the supports can be quite diverse. The intermediate supports are called catwalks, and the abutments are called piers. The abutments serve to connect the bridge with the embankments.
The materials used for bridges are metal (steel and aluminium alloys), reinforced concrete, concrete, natural stone, wood, and ropes.
Parameters of bridges
A bridge diagram is a formula that sequentially presents the dimensions of the design spans – the distances between the centers of the supporting parts of the spans. If several consecutive piers have the same size, their number multiplied by the size of each pier is given. For example (fictional “bridge”), the layout of the bridge 5+3×10+4 m means that the first span of the bridge has a calculated span – 5 meters, the next three – 10 meters each and the fifth – 4 meters.
Bridge construction
The first (and the most expensive – up to 50% of the total construction cost) stage of the bridge is the construction of the piers. The piers are built in open trenches or by sinking piles, lowering wells, caissons or prefabricated casing into the ground. Piles (mostly reinforced concrete) are mainly used in the construction of small and medium-sized bridges. They are sunk into the ground with the help of diesel hammers and electric vibro-loaders. For the construction of large bridges prefabricated casing with a diameter of up to 3 m are mainly used. At present, the most popular pile foundation is the foundation on bored piles (BPP), which are built by drilling in the casing of the inventory pipe. This design is used both onshore and offshore.
The spans are usually set on the supports by assembly cranes. For large bridges, the superstructure is often assembled onshore and then shifted (thrusted) on supports from one bank to the other. The hinged installation method involves extending the structure from the bridge abutment to the span. In this case, the hinged installation with a crane moving on the already built part (for metal spans) or hinged assembly with the manufacture of individual elements in the factory and their subsequent transportation to the site (for reinforced concrete) is used.
Since the mid 90’s of the XX century, the technology of manufacturing plate-ribbed spans of monolithic prestressed reinforced concrete began to be used. This technology has a number of advantages in comparison with the construction of prefabricated spans.
Construction of suspended bridges is done differently: it starts with installation of pylons; then temporary cables are suspended on them.