The first bridges were made by nature — as simple as a log fallen across a stream. The first bridges made by humans were probably spans of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Most of these early bridges could not support heavy weights or withstand strong currents. It was these inadequacies which led to the development of better bridges. The oldest surviving stone bridge in China is the Zhaozhou 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. The first book on bridge engineering was written by Hubert Gautier in 1716. With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

Bridges are classified on the basis that how the four forces namely shear, compression, tension, and moment are distributed in the bridge structure.

Definition

A structure built to span and provide passage over a river, chasm, road, or any other physical hurdle. The function required from the bridge and the area where it is constructed decides the design of the bridge.

Metro construction process is complex, with many construction procedures, long construction periods, and complex technology. Multiple stakeholders are involved in this process, such as contractors, designer, suppliers, and government [4]. Once an accident occurs, the injuries and economic losses are extremely serious. For example, in November 2008, there was a massive collapse at the construction site of Line 1 of the metro in Hangzhou, Zhejiang province, which resulted in 21 deaths and 24 injuries. According to the accident investigation report, the accident was caused by ineffective supervision by the government and the supervisor indulged the contractor’s illegal work; the contractor chose untrained farmers as professional construction workers and did not take proper safety protection measures; and the contractor’s on-site management even did nothing when they identified the precursors of the accident.