Engineering for Scouts 1: Compression, tension, bending and torsion

This series of posts looks at the basics of structural engineering, and how it is useful to you in designing pioneering projects.

A structure needs to be built so that it can deal with the loads that are put on to it. For example, a bridge needs to be strong enough to carry the weight of people walking on it, and a tower needs to stay standing when the wind blows.

To understand how a structure carries these loads, it's important to learn a few terms - actually, you probably know most of these words, but it's important to define them precisely.

Any load on a structure can only do one of four things to it: squash it, stretch it, bend it, or twist it.

Imagine the word above being compressed from either side, making it squeeze together and get narrower. Squashing is called compression. A column holding up a roof, an upright pole in a tent, and a leg of a tripod all carry load in compression.

The word above is being stretched out, with a force pulling on each end of it, making it longer. Stretching is called tension. A guy rope, the rope of an aerial runway, the sheet of a tent and the cables of a tensegrity tower are all in tension.

This word is being pushed down in the middle and pushed up on both ends. Bending is called, uh, bending. The beams of a bridge, the plywood deck of a raft, the throwing arm of a trebuchet and a tall flagpole are all in bending. (yes, a flagpole bends- because the wind blows against it).

The two ends of this word are being twisted in opposite directions.Twisting is called torsion. Torsion can be found in long bridges and tall towers.

These are the basic forces that a structure can experience. The next post will look at how to brace a structure to make it structurally stable (ie to stop it falling over).

... and if you're interested in learning more about structures, then I recommend the book Why Things Don't Fall Down - probably available at your local library, or from Amazon.Com