Early 20th Century pioneering - Manual of Military Engineering

In Scouting For Boys, B-P refers to the Imperial War Office's 1905 Manual of Military Engineering for more information on Pioneering. 106 years later, that same manual is freely available online, and while some chapters are not really relevant to pioneering (eg. table of charges for hasty demolition of walls), there are some great resources, including some camp cooking techniques.

From knots, to lashings, anchorages, bridges and rafts, this book covers a wide range of pioneering projects, and shows how B-P's military experience influenced the sort of activities he chose for Scouting.

Types of pioneered bridge

All bridges can be classified into one of a few categories, based on how the structure supports the load. Pioneered bridges are the same, and most fall into four categories:

1.  Simple or beam bridge: this is the simplest case, a bridge where the deck itself carries the weight of the structure. This bridge built by Culford School Troop is an example, and most drawbridges function this way in the down position.
2. A trussed bridge is a structure where the supports are triangulated members. The double lock trestle bridge is one example.
3. A tied arch bridge uses an arch that hangs above the deck to support the deck. In pioneering, the Banana Bridge uses an arch made from pioneered frames.
4. Suspension bridges use a structure hanging in pure tension to support the load, either as a separate deck like the Abington Spring Bridge, or by walking directly on the tensile members in a monkey bridge.
5. Cable stayed bridges (like the Nelson Mandela Bridge in Johannesburg) use a tower or series of towers that support a deck using straight cables. I'm not aware of any pioneered cabel stayed bridges, but I'd love to be proven wrong.
6. In an arched bridge, an archway forms the deck, or runs under the deck, and supports the load. Ploeg Technieken in Belgium built this bridge supported by arches.

Of these 6 types, types 5 and 6 are pretty rare in pioneering- I can only think of the one arched bridge, and I can't find any cable stayed pioneered bridges. So, before the end of summer (for us down here in the Southern hemisphere) or, while you're waiting for the weather to warm up (for you up there in the Northern hemisphere) , why not give a cable stayed bridge a try?

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

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.

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