bridge

A bridge is a structure designed to span an obstacle, such as a river or valley, allowing vehicles, pedestrians, and other loads to pass across. Most bridges consist of a flat deck, supported by beams, arches, or cables. These structures rest on a foundation that is carefully designed to transfer the weight of the bridge to the subsoil without settling. Bridges can be constructed in a wide variety of forms, determined by the location, intended purpose, and available construction technologies. Simple bridge structures include beam bridges made from logs, and suspension bridges made of ropes or vines. The Romans and ancient Chinese built major arch bridges of timber, stone, and brick. During the Renaissance, advances in science and engineering led to wider bridge spans and more elegant designs. Concrete was perfected in the early nineteenth century, and arch bridges are now built primarily of concrete or steel. With the Industrial Revolution came mass-produced steel, which enabled the creation of more complex forms – including truss and cantilever bridges – that permitted bridges to cross wide rivers or deep valleys. The longest spans use suspension or cable-stayed designs, both of which rely on high-strength steel cables to support the deck. Over time, the maximum achievable span of bridges has steadily increased, reaching 2 kilometers (1.2 miles) in 2022. Other bridge forms include multi-span viaducts, which can cross wide valleys; trestles, a common design for carrying heavy trains; and movable bridges including drawbridges and swing bridges. The design of a bridge must satisfy many requirements, namely connecting to a transportation network, providing adequate clearances, and safely transporting its users. A bridge must be strong enough to support its own weight as well as the weight of the traffic passing over it. It must also tolerate violent, unpredictable stresses imposed by the environment, including winds, floods, and earthquakes. To meet all these goals, bridge engineers typically use limit state design processes and the finite element method. Many bridges are admired for their beauty, and some spectacular bridges serve as iconic landmarks that provide a sense of pride and identity for the local community. In art and literature, bridges are frequently used as metaphors to represent connection or transition. Bridges can create beneficial impacts on a community, including shorter transport times and increased gross domestic product; and also negative effects such as increased pollution and contributions to global warming.