Bridge design
When designing a bridge to traverse a specific obstacle, the designer must identify a design that meets several requirements. The requirements may be categorized as engineering requirements and non-engineering requirements. Engineering requirements include safety, strength, lifespan, climate, traffic, the size and nature of the obstacle to be traversed, and clearance required for passage underneath.
Non-engineering requirements include construction cost, maintenance cost, aesthetics, time available for construction, customer preference, and experience of the builders. Other factors that may be weighed include impact to environment and wildlife; and the bridge's economic, social, and historic relationship to the local community.
Several designs may meet the requirements. After considering all factors, the bridge designerin consultation with the customer will select a particular design. The value engineering methodology can be used to select a final design from multiple alternatives. This methodology evaluates candidate designs based on weighted scores assigned to several different criteria, such as: cost, service life, durability, availability of resources, ease of construction, construction time, and maintenance cost.
Material
Bridges are built from a wide variety of materials, including wood, brick, rope, stone, iron, steel, and concrete. A bridge made from two or more distinct materials is known as a composite bridge.Wood is an inexpensive material that is rarely used for modern motor vehicle roads. Wood is used in bridges primarily in a beam structure or truss structure, and is also used to build huge trestle bridges for railways. When wood is used, it is often in the form of glued laminated timber.
Masonry includes stone and brick, and is suitable only for elements of a bridge that are under compression, since masonry will crack if under tension. Therefore, masonry is limited to structures such as arches or foundations. In the twentieth century, large masonry bridges although superseded by concrete in the Westcontinued to be built in China.
Iron, including cast iron and wrought iron, was used extensively from the late 1700s to late 1800s, primarily for arch and truss structures. Iron is relatively brittle, and has been superseded by the much stronger steel for all but ornamental uses.
File:Reinforcing Steel for Stem Wall at South Abutment .jpg|thumb|This concrete bridge support is being prepared for a concrete pour. The green reinforcing bars will be embedded inside the concrete after the concrete cures.
Steel is one of the most common materials used for modern bridges. Steel was made in small quantities in antiquity, but became widely available in the late 1800s following invention of new smelting processes by Henry Bessemer and William Siemens. Steel is especially useful for bridges, because it is strong in both compression and tension. Steel is widely used for truss bridges and beam bridges, and steel wires are an essential component of virtually all suspension bridges and cable-stayed bridges. Concrete bridges make extensive use of steel, because all concrete used in bridges contains steel reinforcing bars or steel prestressed cables. Steel bridges are more expensive than comparable concrete bridges, but they are much lighter, faster to build, and offer more flexibility during construction and repair.
Concrete is a strong and inexpensive material, but is brittle and can crack when in tension. Concrete is useful for bridge elements that are in compression, such as foundations and arches. Many roadway bridges are built entirely of concrete using a beam structure, often of the box girder variety. Virtually all concrete used in bridges contains steel reinforcing bars, which greatly increase the strength. Reinforcing bars are set inside the concrete form, and the concrete is poured into the form, and cures with the bars inside. If concrete is used in elements that experience tensionsuch as the lower region of a horizontal beam or slab prestressed cables must be embedded within the concrete and tightened. The prestressed cables can be pre-tensioned ; or post-tensioned. The prestressed cables compress the concrete. When the beam is placed into the bridge and carries a load, the undesirable tension normally produced by the tendency of the beam to sag is counteracted by the compression from the prestressed cables. Concrete beams can be precast offsite and transported to the bridge site, or cast in place.
Construction
Elements
The elements of a bridge are generally divided into the superstructure and the substructure. The superstructure consists of most of the visible parts of a bridge, including the horizontal span, deck, wearing surface, trusses, arches, towers, cables, beams, and girders. The substructure consists of the lower portions of the bridge which support the superstructure, including the footings, abutments, piers, pilings, anchorages, and bearings.Footings and abutments are large blocks of reinforced concrete, entirely or partially buried under ground, which support the entire weight of the bridge, and transfer the weight to the subsoil. Abutments are at the ends of a bridge span, where it contacts the subsoil, and sometimes direct the weight diagonally into the subsoil; they also act as retaining walls, keeping the subsoil under the approach road from eroding. Footings are directly underneath towers or piers, and take vertical weight. An anchorage is a massive block, usually made of concrete, that secures the ends of large cables in suspension bridges or cable-stayed bridges. Pilings are strong, lengthy objects placed below footings when the subsoil alone is not sufficient to support the weight of the bridge.
Bearings are mechanical devices placed between the superstructure and substructure which accommodate small rotational or slipping movements that result from thermal expansion and contraction, or minor seismic events.
Substructure
Construction of all bridge types begins by creating the substructure. If the subsoil cannot support the load, pilings must first be driven below those foundation elements. Then, concrete footings are created for abutments, towers, and piers. After the concrete abutments and footings have been created, the piers and pedestals, if any, are built to complete the substructure.When bridge supports are built in a river, lake, or ocean, caissons are often used to provide a workspace while constructing the foundation for the supports. A caisson is a large, watertight, hollow structure, open on the bottom. It is usually sunk to the bottom of the water and workers can work inside, preparing the ground for the foundation. While workers in are inside the caisson, air pressure inside must be kept high to prevent water from seeping in. Workers, if they do not properly decompress when exiting the caisson, can get decompression sickness. Early bridge builders did not understand decompression, and deaths were common: thirteen workers died from decompression sickness when building the Eads Bridge. The entire caisson may be filled with concrete to create the foundation. An alternative to a caisson is a cofferdam, which is a temporary dam surrounding the support location, open on top, where workers may work while constructing the foundation.
Superstructure
After the substructure is complete, the superstructurewhich will rest on the substructure is built.Beam bridge superstructures may be fabricated off-site or cast-in-place. The beams may be laid over the piers by a crane. If the span crosses a deep ravine, a technique known as launching may be used: the full span are assembled on the approach road, then pushed horizontally across the obstacle. Box girders may be created by cantilevering.
Arch bridge superstructure construction depends on the material: for concrete arch, a temporary a falsework forms. Steel arches use the cantilevering method and build each side of the arch outward, joining in the center; temporary piers or falsework may be needed for larger arches.
Cantilever superstructures are usually built incrementally by proceeding outward from anchorages or piers. Most cantilever superstructures can be built without temporary support piers, as the bridge can support itself as it extends outward. A similar process is used for both steel or concrete cantilevers: pre-fabricated sections may be positioned a ground level and hoisted into place with a crane; or maybe transported horizontally along the previously completed portion of the cantilever. Concrete cantilevers require steel prestessing cables to be inserted through tubes inside each section as they are added, and tightened to put the concrete into compression.
Cable-stayed bridge superstructures begin by building one or more towers, which rest directly on footings that are part of the substructure. The deck is constructed in pieces beginning at the tower and moving outward. As each piece of the deck is added, it is connected to towers with steel cables. The deck proceeds outwards in both directions at the same rate, to ensure the forces applied to the tower are balanced. if the deck is made of concrete, steel prestessing cables are inserted through tubes inside each deck section, and tightened to put the concrete into compression.
Suspension bridge superstructures start with the towers and anchorages. The towers may be steel or concrete, and rest directly on the footings. The anchorages are large reinforced concrete blocks solidly anchored into the earth, since they must withstand the pull of the large cables that hold the entire deck and live load. After the towers are completed, a boat carries a rope across the river, and the rope is hoisted to the top of the towers. Then a large wheel is then pulled back and forth across the rope, stringing two wires each pass. After hundreds of journeys, the wires are pressed together to form the cable. The cables are tied to the anchorages at both ends. Vertical wires called hangers are suspended from the cables, and the deck is then attached to the hangers in small sections.