Railway Bridge across Merkys ruins

A bridge is a structure designed to span an obstacle, such as a river or railroad, 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 19th 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, hard-to-predict stresses imposed by the environment, including winds, floods, and earthquakes.