History

• Bridges have long occurred in natural settings. Examples include fallen trees crossing over gullies, and natural arch bridges carved out of stone by a waterway. Ancient man-made bridges modeled natural bridges by combining man-made material (such as iron chains) with natural material (such as bamboo boards). Some of the earliest man-made bridges were built in India in the early 300s. The Roman civilization made notable improvements on the architectural technology behind bridge-building. Early Roman bridges used rudimentary concrete to create long-lasting bridges. Various empires in China also pioneered bridge-building around the same time that the Romans did, but used wood and stone in place of cement. In Central America, indigenous groups incorporated local technology (e.g. ropes) into the creation of bridges. European civilizations later borrowed much from these various bridge techniques and made improvements, with innovations surrounding the newfound use of iron and steel during the Industrial Revolution.
  
  

Significance

• Bridges allow travel over obstacles that would normally force travelers through an extensive detour. Bridges also allow travel to previously unreachable areas. Bridges have played a significant role in the development of national transportation infrastructure in the United States. For example, basic bridge technology allows freeways and highways to be built off the ground in order to pass over residential areas, or even other roads. Major U.S. locales, such as New York City and the Florida Keys, also depend on bridges for much of their transportation needs. Some bridges are also significant because they are used not only for transportation, but to carry electrical utility lines and other forms of power or communication.
  
  

Types

• Bridges can be divided into six major types categorized by how they are built: arch, beam, cable stayed, cantilever, suspension and truss. Beam bridges are the most rudimentary type of bridge, consisting of a horizontal beam placed on two supporting columns. Cantilever bridges are similar to beam bridges but are only supported on one end. Arch bridges are one of the earliest forms of bridge-building technology, and were especially common among early Greek and Roman architects.
  
  

Identification

• A bridge is identified and classified by how it distributes the combined effects of bending, torsion, shear, tension and compression. All bridges will use varying architectural bridge technologies to combat the effects of these external pressures. Bridges are identified by what specific technologies they use. Architects also catalog bridges by their length and span.
  
  

Considerations

• Whether a bridge installation is required or not depends on the bridge's structural and economic efficiency. City engineers and transportation planners consider these two factors to determine whether a bridge should be installed, as well as how to improve a standing bridge. Structural efficiency refers to the bridge's ratio of load (how much weight will be carried by the structure) to physical mass. Economic efficiency considers how much money and resources are saved by having a bridge, compared against the bridge's total cost.