The bridge, generally constructed over rivers, lakes, or seas, is a structure that allows the smooth passage of vehicles and pedestrians.

Evolving to meet the demands of modern, rapidly developing transportation, bridges now also refer to constructions facilitating easier passage across gorges, adverse terrains, or fulfilling other transportation needs.

A bridge typically consists of a superstructure, substructure, bearings, and ancillary structures. The superstructure, also known as the bridge span structure, is the primary structure spanning obstacles; the substructure includes abutments, piers, and foundations; bearings are load-bearing devices supporting the bridge span structure on piers or abutments; ancillary structures encompass elements like bridge decks, tapered embankments, guardrails, and flow control mechanisms.

In ancient times, humans crossed waterways and chasms using fallen trees, natural stone beams or arches, protruding rocks in streams, or vines growing along gorge edges. Determining precisely when humans purposefully felled trees to construct bridges or stacked stones for this purpose is challenging.

The Babylonian Kingdom erected multi-span wooden bridges around 1800 BC, with a bridge length of 183 meters. In 621 BC, ancient Rome built a wooden bridge over the Tiber River, followed by a pontoon bridge across the Strait of Sicily in 481 BC. In ancient Mesopotamia, a stepped stone arch bridge emerged in the 4th century BC. Before the 17th century, bridges were generally constructed using wood or stone materials, categorized as stone bridges or wooden bridges.

The primary form of stone bridge was the stone arch bridge. During the Roman era, Europe saw numerous arch bridges. For instance, between 200 BC and 200 AD, Rome constructed eight stone arch bridges over the Tiber River, with the Fabrizio stone arch bridge from 62 BC having two spans, each spanning 24.4 meters.

In 98 AD, Spain built the Alconétar Bridge, towering at 52 meters. Additionally, several stone arch aqueducts appeared, like the still-standing Gard Aqueduct in France, dating back to the 1st century BC, with seven spans ranging from 16 to 24 meters.

Roman arch bridges were mostly semicircular, with spans less than 25 meters, and had wide piers, about one-third of the span.

The production of iron in the 18th century introduced a new building material for bridges. However, cast iron had poor impact resistance and low tensile strength, making it unsuitable for bridge construction.

After the 1850s, with the development of acid Bessemer and Siemens-Martin steelmaking techniques, steel emerged as a crucial bridge-building material due to its high tensile strength, good impact resistance, and the advent of steel plates and rectangular rolled steel sections in the 1870s, enabling prefabrication of bridge components and widespread steel usage.

In the early 18th century, cement, created by firing a mixture of lime, clay, and red iron ore, was invented. In the 1850s, the use of reinforcing steel in concrete began to compensate for cement's poor tensile strength. Subsequently, reinforced concrete bridges were constructed in the 1870s.

Modern bridge construction spurred the rise and development of bridge engineering theories. In 1857, Squire Whipple presented a comprehensive theory on beams and torsion, building upon previous studies in arch theory, statics, and material mechanics. During this period, theories on continuous and cantilever beams also emerged.

Truss analysis for bridges (such as methods by Warren and Howe) was developed. Post the 1870s, through efforts by Kurt Müller, W.J.M. Rankine, J.C. Maxwell, and others, structural mechanics significantly advanced, enabling stress analysis of bridge components under various loads.

These theories drove the development of trusses, continuous beams, and cantilever beams. By the late 19th century, elastic arch theory had matured, promoting advancements in arch bridges. In the 1920s, the rise of soil mechanics furthered theoretical studies on bridge foundations.

Modern bridges, classified based on building materials, encompass not only wooden and stone bridges but also iron bridges, steel bridges, and reinforced concrete bridges.

Bridges represent a significant reflection of a country's technological advancement and comprehensive strength. Alongside economic development and technological progress, a plethora of world-class bridges now traverse mountains, rivers, and seas, transforming countless obstacles into passageways. Looking ahead, it's hoped that continuous efforts by builders will bring more convenience and abundant happiness to an increasing number of people.