Bridge

A bridge is a structure that crosses an obstacle such as a river, lake, railroad, road, or ravine. Its primary function is to transport vehicles, trains, and pedestrians, but bridges may also accommodate pipelines, buildings, wildlife, and canals. Basic structures used in bridge design include arches, trusses, beams, cantilevers, suspension cables, and cable-stays. Bridges are configured in a variety of forms, such as viaducts, aqueducts, trestles, movable bridges, double-deckers, pontoons, and portable military bridges. They may also be categorized by their materials, which include wood, brick, stone, iron, steel, and concrete.

The history of bridges reflects the evolution of humankind's engineering technologies. The Romans and ancient Chinese built major bridges of stone and timber. During the Renaissance, advances in science and engineering led to wider bridge spans and more elegant designs. Concrete was perfected in the early 1800s, and proved to be superior to stone in many regards. With the Industrial Revolution came mass-produced steel, which enabled the creation of suspension and cable-stayed bridges that could span wide obstacles.

The design of a bridge must satisfy many requirements, such as connecting to a transportation network, providing adequate clearances, and safely transporting its users. Additional factors include cost, aesthetics, and longevity. A bridge must be strong enough to support the weight of the bridge itself, as well as the traffic passing over the bridge. It must also tolerate stresses imposed by the environment, such as wind, snow, earthquakes, water currents, flooding, and temperature fluctuations. To meet all these goals, bridge engineers use analytical methods such as limit state design and finite element method.

Many bridges are admired for their beauty, and some serve important roles as iconic landmarks that provide a sense of pride and identity to a community. Bridges are often used as metaphors in art and literature to represent connection or transition. Bridges can have a significant impacts – both positive and negative – on a community's environment, society, and economy. Positive effects can include shorter transport times, more employment opportunities, increased wages, improved productivity, and increased gross domestic product. Negative impacts of bridges can include contributions to global warming, increased traffic accidents, income inequities, and increased pollution. Provided by Wikipedia