Bridges

A bridge is a structure built to span obstacles such as bodies of water, valleys, or roads to provide passages over the obstacle. There are many different designs of bridges that serve unique purposes and apply to different situations. Designs of bridges differ depending on the function of the bridge, the nature of the terrain, the material used to make it, and the funds available to build it.

The first bridges were made by nature itself. These "bridges" were often trees fallen over that provided a passage across a stream or small river. The greatest bridge builders were the Ancient Romans. They built arch bridges and aqueducts that withstood extreme conditions. Many other civilizations caught on and began to build different types of bridges.

Types of Bridges:

Beam Bridge: horizontal beams that are supported at each end by substructure units. The earliest beam bridges were just simply logs across rivers and other similar simple things. Beam Bridges are the most common bridge type in use today. 

Truss Bridge: A bridge whose load-bearing superstructure is composed of a truss. This truss is formed by triangles. Truss bridges are one of the oldest types of modern bridges. A truss bridge is economical to construct owing to its efficient use of materials.

Cantilever Bridge: They are built using cantilevers-horizontal beams supported only on one side. They are constructed using much of the same materials and techniques as a beam bridge; however, there is a difference in where the forces act. 

Arch Bridge: Arch bridges look slightly like beam bridges; however, they have abutments at each end in which the weight of the bridge is thrust into. Abutments are substructures at the ends of bridges where the bridge's superstructure rests. 

Tied Arch Bridge: They have an arch-shaped superstructure but the ends of the arches are restrained by tension in the bottom of the structure. They are also called bowstring arches. 

Suspension Bridge: They are suspended by cables. In modern suspension bridges, the cables hang from towers that are attached to caissons or cofferdams. 

Cable-stayed Bridge: They are also held by cables. However, there is less cable required to build these and they also tend to be higher. 

Most bridges are fixed bridges, but you can also find movable bridges that having moving parts to it. There are also double decked bridges with two levels to support more traffic. These tend to be in busier areas such as large cities.