Engineering for Scouts 3: Analysing structures

Engineering for Scouts 1 looked at the four different types of forces in a structure, and Engineering for Scouts 2 looked at bracing a structure for stability. This week we'll look at the forces in a structure, understanding where they go and what they're doing to the structure. In these drawings below, structures being compressed are blue, those in tension are red, and those in bending are green. We'll start with the simple flag pole, braced three different ways:

A flagpole with guy ropes running all the way to the top works with the pole in almost pure compression- any time a force acts on the side of the pole, the guy ropes tighten up and take the tension. The stakes at the bottom of each guy rope are bending as they transfer the force from the guy rope into the soil.

If we brace the flagpole with compression struts, then they will all act in compression. If they are staked down, they might act in tension too - any compression strut in a wooden structure can function in tension too, but normally the strut on the opposite side of the structure will be working in compression instead. You can see that if the pole is not braced all the way to the top, the segment that is free will function in bending up to the point where the struts are attached to it.

If you were to dig a hole and bury the end of the flagpole, or hammer a stake in and lash it to the end of the pole, the pole would start off in compression, if you had it balanced perfectly. As soon as the wind blows on it, or it starts to fall over, it starts to act in bending. This is close to how the stem of a tree functions when the tree is alive (except it's anchored by roots, not a stake) and it turns out that whole stem poles (i.e. those that haven't been shaped to size) are very strong in bending to withstand the ordinary forces they would experience.

Finally, a slightly more complex structure - a Kontiki raft:

Firstly, you'll notice that the whole base of the raft is functioning in bending. This is because the deck is functioning like a bridge between the two rows of barrels. You'll also notice that the are no diagonal braces on the base of the raft. Instead, the barrels and the deck are acting like very wide bracing members, stiffening the whole base without needing diagonal members.

If we look at the two end frames, you'll see firstly that they are tied to each other with a rope, which I like to tighten with a trucker's hitch, to make sure that it's properly tight. This ropes is obviously in tension, and the pull at the top puts the top half of each frame into bending, down to the point where it meets the strut that props it up in compression. I have shown the part underneath that in tension, but the reality is that there is a lot of bending going through that bottom half of the 'A' frame as well