Bridge

A bridge is a structure built to span physical obstacles such as rivers, lakes, roads, railways, ravines, or other geographic or artificial barriers. Its primary purpose is to provide passage over the obstacle, facilitating the movement of automobiles, trains, and pedestrians, but it may also accommodate pipelines, utility lines, buildings, wildlife crossings, and even canals.

Bridges are classified into several structural types based on their design and construction. The principal styles include arch, truss, beam, cantilever, suspension, and cable-stayed. Less common forms include moveable bridges, double deck bridges, pontoon bridges, and military bridges. Bridges can also be categorized by their materials, which include wood, brick, stone, iron, steel, and concrete.

The history of bridge building reflects the evolution of engineering knowledge, materials, and construction techniques across different civilizations and eras. The Roman and Chinese civilizations were renowned for their advanced engineering techniques, and some of the bridges they built are still standing, including the 1st century Roman Pont du Gard and the 7th century Chinese Anji Bridge. The Renaissance, in 15th–16th century Europe, brought a new emphasis on science and engineering, leading to stronger bridges with longer spans. With the advent of the Industrial Revolution, iron became an important construction material for bridges. The abundance of inexpensive lumber in Canada and the United States caused timber bridges to be the most common type of bridge in those countries from the late 1700s to the late 1800s. In the late 1800s, concrete replaced stone and masonry as the primary material for bridge foundations. Steel became a common building material for bridges in the late 1800s, leading to suspension bridges and cable-stayed bridges that spanned long distances.

The designer of a bridge must address a wide range of functional, structural, environmental, and aesthetic requirements to ensure safety, efficiency, and durability. Fundamental requirements include structural strength, stability, and the capacity to provide safe passage for users. In addition to these core functions, designers must consider factors such as cost, aesthetics, expected lifespan, the time frame for construction, client or governmental preferences, and the experience and capabilities of the construction team. To address these complex demands, engineers employ methodologies such as Limit State Design, which ensures that bridges remain functional under both normal and extreme conditions. The finite element analysis method enables detailed modeling of stresses and deformations within the structure.

The earliest forms of bridges were simple structures used to cross swamps and creeks, consisting of wooden boardwalks or logs. Pilings – which are critical elements of bridge construction – were used in Switzerland around 4,000 BC to support houses built over water.

Several corbel arch bridges were built c. 13th century BC by the Mycenaean Greece culture, including the Arkadiko Bridge, which is still in existence. In the 7th century BC, Sennacherib constructed stone aqueducts to carry water near the city of Ninevah. One of these aqueducts crossed a small valley at Jerwan with five corbelled arches, and was 280 meters (920 ft) long and 20 meters (66 ft) wide. In Babylonia in 626 BC, a bridge across the Euphrates was built with an estimated length of 120 to 200 meters (390 to 660 ft). In India, the Arthashastra treatise by Kautilya mentions the construction of dams and bridges. Ancient China has an extensive history of bridge construction, including cantilever bridges, rope bridges, and bridges built across floating boats.

The ancient Romans were prodigious bridge builders, renowned for their advanced engineering techniques and durable construction methods. Many Roman aqueducts – some still standing today – used a semicircular arch style. An example is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans used cement as a construction material, which could be mixed with small rocks to form concrete, or mixed with sand to form mortar to join bricks or stones. Some Roman cements, particularly those containing volcanic ash, could be used in underwater applications. The enormous Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.

The oldest surviving stone bridge in China is the Anji Bridge, built from 595 to 605 AD during the Sui dynasty. This bridge is also historically significant as it is the world's oldest open-spandrel stone segmental arch bridge. Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America prior to European colonization in the 16th century.