How+does+a+bridge+support+weight?+Part+2

=﻿= **How does a Bridge support weight?** Kate P. 2011 A bridge is a structure carrying road, path, railroad, or canal across a river, road, railroad, or other obstacle.
 * __What is a bridge?__**

A bridge supports weight when the base of the bridge is made out of strong and good material to support the long span. Bridges can hold heavy loads because of the constant pushing and pulling of each part of the bridge. A suspension bridge suspends its load from main cables that run along both sides of the structure. The weight of the load on the bridge is picked up by the cables and transferred to the bridge’s towers. Bridges are subject to two different forces: compression and tension. While some bridges are simple structures, others are strong and built with strong materials. All bridges must be able to support the weight of the material the bridge is made out of as well as the cars on them. They must also need to get use to the winds and other environmental causes, such as earthquakes. The beam bridge is the oldest and most common type of bridge. A beam bridge is a horizontal structure. It has a support on each end. Other supports, called piers, may also hold up the bridge between the two ends. A log that crosses a stream is a beam bridge in its most basic form. The truss bridge has a support at each end. It may also rest on piers in between. But its structure gives it more strength than a simple beam bridge. A framework of metal or wood bars connects the two ends of the bridge. They often form a sort of tunnel through which the roadway passes.
 * __How does a Bridge support weight?__**
 * __Types of Bridges__**

The cantilever bridge is made up of structures called cantilevers. A cantilever is a beam that has a pier at only one end, like a diving board. A framework of many bars adds strength to the beam, as in a truss bridge. At least two of these beams stretch toward each other to form a cantilever bridge.

Like the beam bridge, the arch bridge is a very old design. An arched structure built beneath the bridge's roadway provides its support. Arch bridges often span rivers and valleys.

In the suspension type of bridge, the roadway hangs from strong wires called cables. The main cables hang between two or more towers. Smaller cables hang down from the curving main cables. Suspension bridges can span longer distances than any other type of modern bridge.

A cable-stayed bridge also uses cables to support the roadway. Its cables run directly between the towers and the roadway. They attach to the roadway in straight, diagonal lines.

Some types of bridges are movable. Some may open upward to allow tall and large ships to pass underneath. Others may turn sideways. Still ones are called pontoons, pntoons float on water.



__**Citation**__ Ardley, Neil. //How Things work.// Pleasantville, New York: Reader’s digest, 2004. Print "Bridge." Britannica Elementary Encyclopedia. Encyclopædia Britannica Online School Edition. Encyclopædia Britannica, Inc., 2011. Web. 22 Sept. 2011. Website 1. Bridge." Compton's by Britannica. Encyclopedia Britannica Online School Edition. Encyclopædia Britannica, Inc., 2011. Web. 21 Sept. 2011
 * __Glossary__**
 * ** Pontoons ** -A flat bottom boat or hollow metal cylinder.
 * ** Compression ** -A squeezing force
 * ** Tension- ** A pulling force
 * ** Span ** -Something, such as a railroad trestle or bridge that extends from one point to another.