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

Show me the Ropes - Knotting instruction videos

'Show me the Ropes' is a series of Youtube videos covering basic knots and lashings, and is being updated all the time. The videos are made by First Walmer, a Scout group in Port Elizabeth, South Africa. From the thumb knot to the Carrick bend, the videos cover a lot of ground.

Simple Friction Lock 'Bridge' - Instructions

I've written about friction lock projects before- like this table and this pole rack. This post a simplified friction locking pioneering bridge.

The purpose of this project is as an 'icebreaker' in a pioneering session - even Scouts who have no knotting experience can build one of these 'bridges', and feel that they have achieved something. A patrol will gain confidence from this exercise that will motivate them to learn lashings during the later parts of the training session.

This project requires 9 Scout staves or similar sized poles (bamboo may be too slippery), and the instructions show a sequence for the construction of the bridge (inspired by my favourite childhood toy, of course). Click on the image to enlarge it, and print the enlarged image. Step 5 is the critical one: your Scouts should pay careful attention to the placement of the two transverse poles, making sure they go over and under the correct poles.

Using only these instructions, all of the patrols in our troop were able to build one of these bridges. The fastest was built in about 2 minutes (admittedly this patrol had built one before) but I would allow 10 minutes for Scouts who have no prior experience. Here is the first patrol finished, with a Scout testing the sturdiness of the structure (please excuse my grainy camera-phone photograph):

SketchUp for Scouts 3: quick lashings in SketchUp

The drawing for this camp couch has a simple square lashing modelled in SketchUp. This tutorial follows on from the previous tutorials on using SketchUp for scouting. If you are new to SketchUp, download it (for free), read the first tutorial on this site and then get going with the square lashing:

Start in Sketchup with 2 poles (at right angles to each other in this case).



Use the rectangle tool to draw a rectangle underneath the 2 poles, then use the PushPull tool to extrude the rectangle up into a box. Using the PushPull tool, resize the box until all six faces if the box (4 sides, the top and bottom) are just touching the poles, like the image on the right above.


Drag a selection window around the box, and then right-click and on the menu that pops up, choose 'intersect with model'. This will add a 'seam' where the poles pass through the box.

Select the two poles and right click to bring up the context menu, then choose hide. (The poles in this image are surrounded by boxes because they are components. Read the first tutorial to see why components are important)


Now that the poles have been hidden, you can concentrate on the rectangular block, which is what your lashing will be made from. Using the erase tool, remove all the lines that are unnecessary, following the steps above to produce a simplified sketch of a square lashing.


When you are done, go to the 'Edit' Menu and unhide 'Last' to show the poles again.

Zoom out and take a look. The lashing wraps around the poles. but is missing frapping turns. Use the line (pencil) tool to draw in the frapping turns, joining the centrepoints of the 4 vertical lines together (the square snap indicator will turn light blue when your mouse is over a centre point)

Select the face created in the middle and hit the delete key to finish your lashing. This sort of lashing might seem very simple when viewed up close but when you zoom out it is quite effective. It is possible to model a lashing in SketchUp with 3 full turns of rope and 3 frapping turns, but aside from the time it would take to make that model, it would also slow your computer's performance down a lot. The method demonstrated here is very kind to your computer, causing almost no loss in speed when working with a model.


A similar technique can be used to make round lashings- the two photographs above should give an indication of how this works.

Sketchup for Scouts 2: Download pioneering components for SketchUp

First, if you haven't heard the good news: SketchUp is now free! If you are using it for fun (ie. not making money out of it) you can now download a free version (19 Mb) at sketchup.google.com - just for PC at the moment but Mac is on the way as well.

The second part of this development from Google is something called the 3d Warehouse, where you can upload and download SketchUp models. Quite a few of my models from this site are up there already for you to download (a small raft, a big raft, a trebuchet and an obstacle course), and I plan on adding more. If you upload your own pioneering projects, tag them with 'scouting' so that they show up in this search.

Sketchup for Scouts: beginning tutorial

Sketchup is the software I use for the drawings on this site. It's simple, fast and produces clear, clean drawings. There is a free version that is unlimited, and a Pro version that adds 3d file export and a few other goodies. So download the latest version here, and once you have got it up and running, join me for a quick tutorial.

I will be looking at drawing the simplest pioneering structure, the tripod.In the next few drawings I will be introducing you to most of the basic tools in SketchUp, and some of the most important concepts (click on any of the pictures to see a full-screen view of it).


Open Sketchup and look in your toolbars for the circle tool. Click on it, then move your mouse over the white area in the middle of the page. Your circle should turn blue- this means that it is going to draw flat on the floor (Sketchup uses 3 colours- red, blue, and green, to represent different directions. Blue is flat, green is vertical north-south, and red is vertical east-west. This way, anytime a line turns a certain colour, you know it is lined up with that direction). Click a point to select the centre of your circle, then move the mouse and watch how the circle gets bigger and smaller. Down in the bottom right hand corner of the page is a little box with a number in it that changes as you move- this box is called the Value Control Box, and at the moment is showing the radius of the circle. We want the radius to be 100mm (4 inches if you are working in Imperial measurements) so you can either move the mouse until this value reads properly, or you can do this:just click anywhere to make a circle of any size, then type in '100mm'and hit enter. The circle will resize to the number you typed. This works for almost every command in SketchUp that takes a number as an input. This only works when it is the VERY NEXT THING you do after placing an object.

One of the cleverest thigns about SketchUp is how you navigate in 3d space. If you are using a mouse with a scrollwheel, roll the wheel up and down. This ZOOMS in and out. Now hold the mouse wheel down and drag the mouse. This ORBITS or moves your eye around the model. Now hold SHIFT + the middle mouse button and drag- this PANS the page around. Move around like this a bit to get comfortable. IF YOU WANT TO GET THE MOST OUT OF SKETCHUP, YOU NEED TO ORBIT,ZOOM AND PAN CONTINUOUSLY. As you are drawing, these tools are available- you can always zoom in and out, orbit and pan- even while you have placed a circle and are still sizing it. Next, let's turn our flat circle in to a 3 dimensional drawing:




Look for the tool that has a box with an arrow pointing up. This is the push-pull tool, and you will spend a LOT of time in SketchUp using this tool. Click on the tool, then click on your circle, release, and move the mouse. Watch how the flat circle is turned into a pole ( I hope that answers your question, Meydad). I want a 2m long pole, so I will just click the mouse to end the pole, and type in '2000mm' to set the height.



A very important discipline to learn in SketchUp is to group everything together. At the moment, if you click on your pole, you will see that the part you click on is highlighted- click the circle on top, it highlights. Click on the black edge, and it highlights (by turning yellow). We want to be able to easily select the WHOLE object and move it around. So a few tips: Double-clicking any entity (a line or a face) will select that entity and the edges or faces around it (double click on the middle of the circle on top of the pole and you will see the line around the edge selects as well); Triple clicking selects all of the entities connected to it. Triple-click to select the whole pole (top,bottom and the line around them) then right-click. We are going to look for an option called 'make component'and click on it.

A component is one way of joining entities together to make it easier to work with them. Give your component a name, and make sure that 'replace selection with component' at the bottom is selected. Components are very similar to another collection of entities in Sketchup called a group. This is the difference: every component in a drawing is EXACTLY the same as every other component with the same name. So if you make the pole a component, then make a few copies of it (I'll show you now how to do that...), when you go and change the length of one pole, all of the others will change. You would use groups if you don't want that to happen (there are other ways around it, but for today, that's a good enough explanation). So, make a component from the pole, and remember that components are all copies of each other, and changing one will change all of them.



The move tool is next. This tool is a bit sneaky, because it is actually move, copy and rotate all rolled into one tool. Click on an object and move the mouse to move it- notice how SketchUp tries to keep you in the red,green or blue directions- this helps you keep your drawing accurate. New hit escape and try holding down 'ctrl'and clicking on an object (I think that's option-click on a Mac). You should see your cursor adds a small + to it- this means you are in 'copy' mode. The final thing this tool does is rotate. That's what we'll use it for right now- select the move tool, and move the cursor over the pole. You'll see a yellow box around it, and 4 red + marks. When you move the cursor over one of these, a protractor (circle with angles marked on it) pops up on that box face. Click and drag to rotate the pole.




The last new tool to learn(for this lesson) is the 'rotate' tool. Select the pole, then click on the rotate tool (try clicking on the rotate tool without selecting anything to see how it reminds you what you are doing wrong). Like the circle tool,the rotate tool also tries to guess which direction you are working in- it should turn blue as you move it onto the area under the pole. Like the move tool, the rotate tool ALSO has a copy tool hidden in it- move the protractor under the pole and ctrl-click it (making sure the + appears on the cursor). Now when you move the mouse you should see a second pole rotating around. Place it and then type in '120' to set the rotation angle to 120 degrees. We want to actually add two poles, not just the one, so type in '2x', and you'll see another pole appear (this trick also works for move-copying)


We are nearly done- you should have a tripod of some kind- but maybe like mine the poles pass through each other, or maybe they miss altogether. This is not good pioneering :), so we will try to fix that.


Double-click on one of the poles- notice how the other poles have faded away a bit and the box around the pole is dotted. You are now working 'inside the component', you can always enter any component any time in Sketchup to do this. Still working inside the component, triple-click it to select all the parts, and select the move tool. Now as you move the pole around, notice that the other 2 poles move as well- because they are all copies of the same component.


Move the pole until the three are just touching, then hit escape to close the component.




Almost done- the last thing to do is to select all the poles- click the select tool ( the cursor arrow) and draw a window around them. Make sure all three are selected, then right-click and choose 'Make Group'.


Now that you have made a group, you can select it and copy it (by using ctrl-move) to line 3 tripods up like this.

And that's your first SketchUp model done! Plus you know tools that you will be using over half the time you are working in SketchUp.

There are a few resources you can look at for more help:
-Print out the quick reference card (for PC or Mac) and keep it next to your computer all the time.
-Watch the video tutorials if you have broadband- this is the BEST way to learn SketchUp.
-Check out the self-paced tutorials that come with SketchUp.
-SketchUcation is a website with more advanced tutorials, a good place to learn some nice tricks, especially if you want to be able to work QUICKLY in SketchUp.
-Finally, join up at the SketchUp official forums and ask for help there- people are VERY helpful and you will have an answer to just about any SketchUp related question in a few minutes. One tip- please do a search on the forum to see if your question has already been answered.

Learn SketchUp

SketchUp is a free 3d modelling program by Google. The computer models on this site are made with the pro version of this program.

Some lessons for SketchUp, concentrating on Scout pioneering:

SketchUp Basics - Introduction to SketchUp- modelling a tripod

Scouting models - Download SketchUp models of projects featured on this site

Modelling lashings in SketchUp - Tutorial for making simple square and tripod lashings for your pioneering models in SketchUp.

There are a few resources you can look at for more help:
-Print out the quick reference card (for PC or Mac) and keep it next to your computer all the time.
-Watch the video tutorials if you have broadband- this is the BEST way to learn SketchUp.
-Check out the self-paced tutorials that come with SketchUp.
-SketchUcation is a website with more advanced tutorials, a good place to learn some nice tricks, especially if you want to be able to work QUICKLY in SketchUp.
-Finally, join up at the Google SketchUp official forums