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August 23, 2013

Engineering for Scouts 2: Bracing structures

Engineering for Scouts 1 covered tension,compression,bending and torsion - the different types of force that a structure can experience. This post looks at how structures can be braced to make them stable.

So, if we start with the simplest possible project, a flagpole, we will see there are three things we can do to brace it so it stands: we can put guy ropes onto it (tension bracing), put poles onto it to prop it up (compression struts) or anchor it into the ground, either by heeling it in, or hammering in a stake and lashing it to the stake. These correspond to three of the forces we looked at last week: guy ropes work in tension, props work in compression, and a stake works in bending.

More complex structure, made of more than one pole, fail in a few different ways: Parallelolgraming and torsion can be solved with bracing, and joint failure can be solved by controlling lashing quality (tightness, as well as condition of the ropes used), and the breaking of spars can be prevented by maintaining equipment and using the right size spar.

You might have noticed that no matter how tightly you tie a square lashing, no matter how many turns and frapping turns you use, it is still possible to move the two poles with respect to each other, like a pair of scissors. This kind of connection, that is not perfectly stiff, is called a pin joint. If you were to make a square out of pioneering poles, with square lashings at each corner, it would collapse sideways very quickly. Engineers call this kind of collapse "parallelogram failure". If you look at a triangular frame, you will see that it is impossible for it to parallelogram, even if the joints are all pin joints. So we can make a rectangular structure stronger by adding extra members to it, to make it into a series of triangles. Engineers call this kind of structure a triangulated structure, and this is the most common way of strengthening a structure with pin joints.

Bracing can be with tension cables (ropes), in which case you need a pair of them, so that one of them is always in tension to keep the structure square, or a single compression pole: if pushed in one direction, it will work in tension, in the other direction it will work in compression.

The closer the brace is to being at the corner, the better. Looking at this series, the one on the left is the strongest brace, and the one on the right is the weakest.

If you look at structures in Scouting, and in the wider world, you will see bracing used like this all over to make the structures stable and strong.

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