By Michael Morrissey

http://www.howstuffworks.com

There's no doubt you've seen a bridge, and it's almost as likely that you've traveled over one. If you've ever laid a plank or log down over a stream to keep from getting wet, you've even constructed a bridge. Bridges are truly ubiquitous - a natural part of everyday life. A bridge provides passage over some sort of obstacle: a river, a valley, a road, a set of railroad tracks...

In this article, we will look at the three major types of bridges so that you can understand how each one works. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use, and by the time you are done reading this article you will understand why. (670 no spaces)

The Basics

There are three major types of bridges:

· The beam bridge

· The arch bridge

· The suspension bridge

The biggest difference between the three is the distances they can cross in a single span. A span is the distance between two bridge supports. A modern beam bridge, for instance, is likely to span a distance of up to 60 meters, while a modern arch can safely span up from 240 to 300 m. A suspension bridge, the pinnacle of bridge technology, is capable of spanning up to 2,100 m. (399 no spaces)

What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with two important forces called compression and tension:

Compression is a force that acts to compress or shorten the thing it is acting on.

Tension is a force that acts to expand or lengthen the thing it is acting on. (339 no spaces)

A simple, everyday example of compression and tension is a spring. When we press down, or push the two ends of the spring together, we compress it. The force of compression shortens the spring. When we pull up, or pull apart the two ends, we create tension in the spring. The force of tension lengthens the spring. (258 no spaces)

Compression and tension are present in all bridges, and it's the job of the bridge design to handle these forces without mistakes. Buckling is what happens when the force of compression overcomes an object's ability to handle compression, and snapping is what happens when the force of tension overcomes an object's ability to handle tension. The best way to deal with these forces is to either dissipate them or transfer them. To dissipate force is to spread it out over a greater area, so that no one spot has to bear the brunt of the concentrated force. To transfer force is to move it from an area of weakness to an area of strength, an area designed to handle the force. An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference. (646 no spaces)

Truss Strength

A single beam spanning any distance experiences compression and tension. The very top of the beam experiences the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. (236 no spaces)

If the beam were designed so that there was more material on the top and bottom, and less in the middle, it would be better able to handle the forces of compression and tension. (144 no spaces)

In addition to the above-mentioned effect of a truss system, there is another reason why a truss is more rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area. (311 no spaces)

Total: 3 139 no spaces

TASKS

1. Give the definition to the next words/word combinations:

a) spring;

b) the suspension bridge;

c) compression;

d) tension;

e) buckling;

f) span;

g) size of the obstacle.

2. Tell about one of the following bridges:

a) The Akashi-Kaikyō Bridge, Japan;

b) The Golden Gate Bridge, USA;

c) The Helix Bridge, Singapore;

d) The Rolling Bridge, England;

e) Langkawi Sky Bridge, Malaysia;

f) Octavio Frias De Oliveira Bridge, Brazil;

g) Henderson Waves Bridge, Singapore.

3. Answer the following questions:

a) What is the bridge?

b) What kind of bridges do you know (except three major types, listed in the text)?

c) Describe advantages and disadvantages of each type of bridges.