BRIDGE-DESIGN

There's no doubt you've seen a bridge, and it's just 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 railway.

The choice of bridge design depends upon the size of the obstacle;
How far is it from one side to the other.
This is the major factor in determining what type of bridge to use.

A modern beam bridge, for instance, is likely to span a distance of up to 200 feet.
A modern arch can safely span up to 800 or 1,000 feet.
A suspension bridge, the pinnacle of bridge technology, is capable of spanning up to 7,000 feet.

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 are two important forces called compression and tension:

Compression
- is a force that acts to compress or shorten the material it is acting upon.
Tension - is a force that acts to expand or lengthen the material it is acting upon.

A simple, everyday example of compression and tension is a coil spring.
When we press it down, or push the two ends of the spring together, we compress it.
The force of compression shortens the spring.
When we pull the spring up, or stretch it apart, we create tension.
The force of tension lengthens the spring.

Compression and tension are present in all bridges, and it's the job of the bridge to handle these forces without buckling or snapping.
Buckling
is what happens when the force of compression is too great to handle.
S
napping is what happens when the force of tension is too great to handle.

The best way to deal with these forces is to either dissipate, or transfer them.
To dissipate a force is to spread it out over a greater area, so that no one spot has to bear the whole brunt.
To transfer a force is to move it from an area of weakness to an area of strength;
An area designed to handle it.

An arch bridge is a good example of dissipation;
A suspension bridge is a good example of transference.

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