Bridges are structures used by people and vehicles to cross areas that are obstacles to travel. Engineers build bridges over lakes, rivers, canyons, and busy highways and railroad tracks. Without bridges, people would need boats to cross waterways and would have to travel around such obstacles as canyons and ravines.
Bridges range in length from a few feet or meters to several miles or kilometers. A bridge must be strong enough to support its own weight as well as the weight of the people and vehicles that use it. It also must resist natural occurrences, including earthquakes, strong winds, and changes in temperature. Most modern bridges have a concrete, steel, or wood frame work and an asphalt or concrete roadway.
Most bridges are held by at least two supports set in the ground. The distance between two adjacent supports is called a span of a bridge. The supports at each end of the bridge are called abutments, and the supports that stand between the abutments are called piers. The total length of the bridge is the distance between the abutments.
Most short bridges are supported only by abutments and are known as single-span bridges. Bridges that have one or more piers in addition to the abutments are called multi-span bridges. Most long bridges are multi-span bridges. The main span is the longest.
There are seven main kinds of bridges
 | Cable-Stayed Bridge | 5 |  | Girder-bridge | 1 |
 | Suspension Bridge | 6 |  | Truss Bridge | 2 |
 | Movable Bridge | 7 |  | Arch Bridge | 3 |
 | pontoon bridge | -- |  | Cantilever Bridge | 4 |
The types of bridges vary in total length. The length of their spans, And the weight they can support. Before deciding which kind to build at a particular place, engineers determine the length of the structure and of each span. They also must consider the maximum load that the bridge will carry and the materials available to construct the bridge.
