- Highway bridges are built to carry vehicles across obstacles that impede a roadway. Highway bridges often carry traffic over water, but may also be utilized to allow the crossing of railroad tracks, or other roadways. Expressway interchange systems also incorporate highway bridges to connect intersecting roadways. In some cases, bridges are used as part of an exit ramp structure allowing the smooth transition of traffic from a high-speed expressway to a roadway designed for slower speeds. Common bridge building materials include reinforced concrete and steel. Various types of highway bridges are called overheads, underpasses, overpassings and viaducts, depending on use.
- A railroad bridge is similar to a highway bridge in basic design; however, railroad bridges are constructed to carry a train over an obstacle. A railroad bridge must withstand the heavy load of a train. In addition, these bridges must often withstand the extra weight of the roadbed material, usually crushed rock or stones, used to support iron tracks. Many railroad bridges are designed with iron plates or concrete slabs on the underside of the structure. This is often necessary to prevent roadbed material from falling to the surface below the bridge. Because of the increased weight these structures must support, railroad bridges are often constructed with welded or bolted steel plate girders. Pre-stressed or reinforced concrete is also a common material used to build railroad bridges.
- Pedestrian bridges are solely used by people to cross over roadways or other obstacles that would otherwise be difficult or dangerous to cross. These types of bridges are common in urban areas where highway bridges are spaced too far apart to allow people easy access to walk between neighborhoods. Additionally, many urban expressway designs incorporate a center median wall, making pedestrian crossing impossible on foot or via bicycle. Pedestrian bridges also are used to enable people to safely cross over busy, multiple lane surface streets in large cities. These types of bridges are also used in rural areas to assist people engaged in recreational activities, such as hiking, biking, and horseback riding cross rivers and natural contours in the land. In some cases, rural pedestrian bridges are designed to allow small off-road vehicles, like ATVs, to cross obstacles. Pedestrian bridges are often built with a variety of materials including concrete, steel or wood.
- Bridges are built in a wide range of designs, however; all bridges are constructed with basic types of configurations. Simple bridge types use girders or beams to hold the bridge deck. Truss bridges are built using a framework of steel or wood to support the structure's deck. Arch-type structures have curved framework to provide support and often incorporate trusses. Cantilever bridges are made of a series of trusses, girders or beams. These types have an arching structure in the center of the bridge connected to supports, with additional pieces that form a half arch at each end. Cantilever designs are often used for bridges that must span longer distances than simpler designs. Suspension bridges use cables, held up by tower structures, to hold the bridge deck in place. The longest bridges ever constructed are suspension type structures.
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