General Discussion

[fishman100]

Semi off-topic question: How would you build your own testing platform? I've never understood how to cut a square in the center without entry from a side. Would you use an exacto knife to score it and then cut it with a stronger knife...?

[chalker]

Semi off-topic question: How would you build your own testing platform? I've never understood how to cut a square in the center without entry from a side. Would you use an exacto knife to score it and then cut it with a stronger knife...?

There are several ways to do it. Some saws can do something call 'plunge cutting'. The easiest way is probably just to use a dril to drill 4 holes in the corners, then a jig saw to cut the edges between the holes. You can also use the jigsaw to 'square' the corner holes afterwards.

[LKN]

Thanks for the explanation Balsa Man. Would you recommend dense balsa at less than 1/64 or possibly even bass at 1/64. I think that using such thin (<1/64) wood strips, bass be easier to apply, and possibly more effective, for lamination?

[Balsa Man]

Thanks for the explanation Balsa Man. Would you recommend dense balsa at less than 1/64 or possibly even bass at 1/64. I think that using such thin (<1/64) wood strips, bass be easier to apply, and possibly more effective, for lamination?

Glad to be able to provide some insight.

Personal opinion, only- others may see it differently-
I'd lean to balsa. 1) 1/64th balsa sheet is readily available; it may be out there, but I'm not aware of a source of 1/64th bass sheet; 2) the density range in balsa is a lot higher than in bass 3) balsa at the high end of the density range is amazingly strong; 4) limited data, but from some tension testing with bass, spruce, and high-density last time boomilevers were up, at a given weight, balsa did better in tension. Now in a lam, that's only a part of the picture, so take it for what its worth.
Two ways I've used to go down in thickness; sanding after you've cut strips, and a planer - a machine that planes. Sanding, you glue one end to flat piece of board, and sand in one direction- away from the glued-down end with sandpaper on a block. Its tricky to keep the samding block level (i.e., not to get some rounding of the surface you're sanding); that's not all that big a deal. You can get a flat surface by putting down a couple strips on either side of the sanded strip, and setting up your sanding block so the block extends past the edges of the sandpaper- the block rides on the strips. You'll have to diddle with the thickness of the ride strips to get the thickness you sand down to where you want it.

A few more ideas/points to keep in mind. By putting thin strip lams on the side(s) of a core stick, you are setting up a preferential failure direction. For a single side lam on a square cross-section, preferential failure will be on/twoard the side away from the lam. On an angle-iron lam, it will be on/twoard the diagonal corner opposite the diagonal corner where the lam strips meet. Angle-iron lamination can add significant strength even when the strip width is significantly less than the width of the core (e.g.1/16th onto 3/32nds) Doing angle-iron lams, getting the edges (on the diagonal corner that the lams are on either side of) glued really increases the strength. Best way I know to get this right is to use a strip on one side that is wider than you want to end up with. Glue it on so that one edge extends past the diagonal corner between the two sides you're laminating to. Glue the adjacent side so its edge is in full contact with the first lam. Trim the excess/overhang after the glue has set up.
Have fun.

[Balsa Man]

Semi off-topic question: How would you build your own testing platform? I've never understood how to cut a square in the center without entry from a side. Would you use an exacto knife to score it and then cut it with a stronger knife...?

There are several ways to do it. Some saws can do something call 'plunge cutting'. The easiest way is probably just to use a dril to drill 4 holes in the corners, then a jig saw to cut the edges between the holes. You can also use the jigsaw to 'square' the corner holes afterwards.

Drilling corner holes and using a saber saw works fine, and is easy.
The shape/size of your hole really doesn't matter, within reasonable limits. What does matter is that you have accurately marked the rules dimensions (i.e., a 20x20cm square), and your hole is in the center, and big enough for the chain to get thru with decent clearance; circular hole cutter, oh, 2-3 inches works great. You want your "rules square accurately marked so you can be sure your legs will clear it at actual competition, and if your base/bottom of legs dimensions are close (which you do want to minimize the lean-in angle of the base legs), this marking will give you practice setting the tower up for competition

[jma]

Thank you Mr. Joeris for informations about the loading block.
I read through all the tower building forums but didn't see any discussion of connecting the chimney to the base for the square base tower. My friend and I are sixth graders and this is our first time to compete in this event so we decided to go with a square base since it's more stable than the retangle one. I looked at the towers from previous competitions on youtube. Some towers have a double crossing bar ( one on top of the base and one on the bottom of the chimney) and some have single where the chimney and the base are connected. What are the good/bad things for each of them? If I go with the single one, do I connect the chimney's legs directly to the base's legs?

[Balsa Man]

Thank you Mr. Joeris for informations about the loading block.
I read through all the tower building forums but didn't see any discussion of connecting the chimney to the base for the square base tower. My friend and I are sixth graders and this is our first time to compete in this event so we decided to go with a square base since it's more stable than the retangle one. I looked at the towers from previous competitions on youtube. Some towers have a double crossing bar ( one on top of the base and one on the bottom of the chimney) and some have single where the chimney and the base are connected. What are the good/bad things for each of them? If I go with the single one, do I connect the chimney's legs directly to the base's legs?

jma, I'm glad the info on the block was helpful.

I’ll give you my thoughts on your specific questions in a minute, but first a few thoughts and concepts for beginners that might help you. A lot of the info and thoughts I post are directed at the.....little details that get you those last couple grams, or fractions of a gram off- ways we have figured out over the years. This kind of information assumes a level of experience and building skills that it takes time to develop. To someone who has never done the tower (or bridges, or boomilevers) event, I’m sure a lot of it sounds like Greek. So this morning, a quick review of some basics – what’s this all about, and how do you best approach it.

These thoughts – like everything I post – are simply from our experience and what we’ve learned from trying a lot of different things; some that worked out great, some that didn’t; each year I've been coaching, we learn new things. I try to share that learning here. Do I think I have all the answers/know the best way to do things; not at all. I'm not an engineer, but I have learned a lot about engineering over the years from a number of good engineers. The structures, every year, that win/medal at Nationals are a) amazing, b) VERY well engineered, c) very well built, and d) have involved/required a LOT of time. One year, when the event was bridges, where you found out after you checked in whether you had to load it at the middle or over toward one end, the efficiency of our bridge was, as best I could tell, up in the Nationals medal range (our team did not go to Nationals, though). That is our high-point so far. We pretty much consistently win at the Regional level, and regularly win/medal at State. We’ve learned some things- some basic approaches – that will get good results, and we’ve learned how to be efficient with our time; how to get to good results with a minimum of time investment. Do understand, though, 1) that to do at all well, it IS going to take a lot of hours, and 2) you are not likely to get to "that winning level" your first, or even your secind year. Enough context...

Wood, basic terminology, shape, and forces:What you are trying to do is come up with a design where you have the least and/or lightest wood put together in a way that it is just barely able to hold the max load of 15kg. There are lots of different ways to get there - choices– trade-offs.

Wood varies in strength. In general, the denser the wood, the stronger. Balsa has a VERY wide range of densities, from crazy light, soft and floppy, to really stiff and strong, with a density about like, say pine wood. Bass wood has a narrower range of densities; that range is within, and toward the high side of balsa’s range. In a loaded structure, like a tower, “members” (the individual pieces that make up the structure) are under either compression or tension forces. Compression is where the forces from loading the structure are pushing on the ends of the member- along the member’s long axis. Tension is where the forces from loading are pulling along the member’s long axis.

In a tower, be it 3 legs or 4, the legs are under a compression load; the weight hanging from the load block (which sits on the top ends of the legs) is equally distributed between the legs. The force from the loading block is straight down. That force goes down through the legs.

In the upper section of the tower, where the legs are pretty much straight up and down/vertical, the compression force the legs are under is about 1/4th (or 1/3rd for a 3-leg tower) the load; a 15kg load on the block means a 3.75kg force on each leg in a 4-leg tower. In the lower (base) part of the tower, the legs are angled out more. Because of this, the compressive force they experience (from a vertical load) is more than if they were straight up and down. Mathematically, that force (for a given load) increases proportionally to 1 over the cosine of the angle away from vertical (talk to your math teacher on what that’s all about/what it means). What it tells us is that for a 4-legged, B-Division tower, the bottom legs are at about a 12.5 degree angle, instead of seeing a 3.75kg force from a 15kg load, they each see a force of about 3.84kg. Not a lot more than in the upper section legs, but a little. We’ll refer to these forces as our “design forces” a bit later in this discussion.

The rules essentially define the overall shape and appearance of the tower. At the bottom, the leg ends have to clear- be outside of –the test base hole, which is 20 x 20cm. You want a few milimeters clearance. At the top of the base section (at a height of 30cm above the loading surface), the leg ends need to clear - fit inside of - a circle 8cm in diameter. You want maybe a milimeter; no more than 2mm clearance. For a 4-leg tower with a square base, those leg positions form a truncated pyramid (a pyramid with the top cut off). The edges/corners of the pyramid lean in toward the vertical centreline at an angle of approximately 12.8 degrees. The shape of the top part is also defined. The bottom ends of the legs in the top part need to line up exactly with, and sit on top of the top ends of the legs in the bottom part. The top ends of the legs in the upper part need to fit under the corners of the 5cm x 5cm square load block. It is possible to fit legs outside the test base hole, inside the 8cm circle at 30 cm above the test base, and under the corners of the load block with single piece, curved legs. For beginning builders, though. Let’s stay away from that more complex option, so we are talking a 2-piece tower; the lower “base” section, and the upper, “chimney” part. The base part is a truncated pyramid with a height of a bit under 30cm- I’d recommend 2 or 3mm less than 30cm. That way, it is the lower end of the top section legs that the 8cm clearance circle has to fit around. Again, with this 2-piece configuration, you have to build it so that the bottom ends of the top legs fit right on top of the top ends of the lower legs.

In the upper section, which is long and skinny (compared to the base section), the dimensions for the leg end locations are set (inside the 8cm circle, and under the corners of the 5cmx5cm load block. The question is how tall? The rules say between 40 cm and 70cm for the tower. For anyone with good building skills, who did a tower last year, I’d say go for 70cm. The rules give a significant advantage for going to max height. For a beginner, you may want to 40, maybe 50 cm. The reason for this suggestion is, the taller you go, the more precise your build is going to have to be to “work.” By “work”, I mean not fall apart/fall over at a pretty low weight loading.

Let’s talk about “precise” for a minute. We’ve talked above about the “theoretical shape”- the lines the legs have to follow between a set of locational dimensions at three levels/planes- on the test base, at 30cm above the test base, and at the top of the tower. With a magic drawing program, or even carefully hand drawn, you can “put those imaginary, perfect lines on paper; define them. That defines the “perfect” or theoretical dimensions – the shape of the tower. Everything is symmetrical around the vertical centreline.

The trick – the big part of this event – is how you actually manage to build a structure where the legs are very close to being right on these theoretical lines. The precision of how you do this is important because if you get “perfect” actual positioning, then the forces on each leg are equal, and they are only the design forces we talked about earlier. The wood you use will only need to be strong/heavy enough to carry these design forces for the tower to carry full load. If one or more of the legs is “out of alignment”, and/or if the tower is not “straight up and down”, then one leg (or maybe 2) will see “more than its share” of force. It doesn’t take much mis-alignment for actual forces to get well above your perfect, design forces. So if the wood you’re using is just strong enough to carry your design forces (when in “perfect” alignment), it won’t be strong enough to deal with the extra forces resulting from mis-alignment. When this happens, the overloaded leg breaks, and down comes the tower. The more things are “out of alignment”, the stronger (and that means heavier) the legs need to be to carry “extra” force beyond the “design” force. “Precise” is an imprecise term- as in how precise is really precise; as in how precise is “good enough” for a tower. I can tell you four things – 1) the more precise you can get, the higher efficiency tower you will be able to build, 2) if the location of the leg ends (relative to your theoretical design location/alignment) is off by much more than 1/16th inch either way you are going to have major problems getting to a decent tower, 3) It is quite possible and relatively easy if you are careful and patient to get better than 1mm precision; it is possible to get to a fraction of a millimeter (one or two tenths- but don’t expect this until you’ve done this for a couple of years or more), and 4) the single place where high precision IS necessary is getting the bottom of the top legs to sit right on top of the tops of the bottom legs - a millimeter off here, and you are going to have major problems

The way to “get things to line up” is some kind of 3-dimensional way to hold the legs in the right positions – our theoretical design alignment. Actually, we need two such “things”- one for the upper section; one for the lower section. The right term for these “things” is “jigs.” It is possible to build a one-piece jig for the whole tower. My experience is, that is harder than doing two pieces. The way you use a jig is it holds the legs in-place, and then you glue the bracing in between the held legs; that holds them together, then you put the two tower sections together to get a complete tower. There are a lot of ways to make a jig. Check out Youtube – there are a couple videos out there about this. Review the various discussions on this forum for ideas. From the ideas you see and get, figure out what you are comfortable trying - at what level you want to try to take this challenge on - how much time you have to devote to S-O, and to this event. The time put into building the best jig you can will really pay off for the rest of the season. We generally spend the first 2-3 months of the season getting good jigs done- start actual structure building in january.

The other critical thing that using a decent jig gets you is repeatability of shape. Here’s why that’s so important. Say you build a tower (without decent jigging), using some weight/size of wood- you test it, it fails at 7kg. You build another, this time using, say, 25% heavier leg wood, or a tighter bracing interval (see below); you test it, and it fails at 9kg. If the SHAPE is not the same, you have no idea why it failed- was it because you needed 50% heavier wood, or because one leg was out of alignment enough that it was seeing 40% of the load instead of 25%. Unless you can build the “next” tower the same shape as the previous one, you will be shooting in the dark trying to get to what is the minimum strength/weight wood you need for it to “work.” You will be much better off with a jig that gives you the same shape time after time, even if it is “off in precision” within reasonable limits- maybe as much as 1/10th, even 1/8th of an inch. At least if you have the same shape, you can get to wood strong enough to just hold the “unbalanced” forces resulting from things being out of alignment.

The legs as columns, and how columns work
The legs are long columns, and there is some basic engineering and math behind how columns work. I’ve discussed this in previous posts in this thread- re-read that carefully. The five important points to understand are:
1) for a given size and density of wood weight, the compression force that piece of wood will carry (before it “buckles” depends on its length. If it carries 4 kg at a length of 10cm, at 5cm it will carry 16kg, and at a length of 20cm, it will carry 1kg.;
2) By putting bracing along the legs – “ladder” pieces – those “crossing bars” you refer to, along with diagonal braces between the ladders (“X” or “Z” bracing), you “lock” the point of the legs where the bracing attaches into a point in space- you make it where it can’t move. The sections between bracing – the “braced interval”, or “exposed column section” become “the effective column length”; their strength resisting buckling failure will be the same as a bare piece at that effective column length;
3) By putting bracing between the legs (at some even interval), you end up with a long column (the whole leg piece) divided up into a series of shorter columns that can carry a lot more compression loading than the long column would without bracing
4) You can get stiffer/stronger legs by either “beefing up” the leg wood (bigger cross-section, higher density), or by tightening down your “bracing interval”- shortening the “exposed column length” between bracing levels. Part of “design” is how you balance/trade-off heavier legs with less bracing, vs. Lighter legs with more bracing.
5) Except for the ladder at the top of your bottom tower section, the forces on your bracing, compared to the compression forces on your legs is quite small- the bracing can be quite light, compared to the legs. This is very true when the tower has been built precisely. As you get less precise, and things are more “out of alignment” to start with, the loads on the bracing become significant earlier in the loading process, and if the bracing isn’t strong enough, the tower will break. When the tower is “precise”, the bracing only has to resist the beginning of buckling failure, when the braced point of the leg first starts to buckle in or out- that first tiny bit – like a thousandths of an inch - of bending puts very little force on the bracing- a little bit of bracing keeps it from moving further – you still have an intact column.

And last, your specific questions.
You are on the right track with a square base; that’s how you get the shortest legs to span the test base opening- you locate the leg bottoms at the midpoints of the four sides of the opening., and a square gets you shorter/less bracing wood in the bottom section.

As to whether you want a ladder (which is what those “crossing bars” you describe are) both at the bottom of the top section, and the top of the bottom section; I’d say no. We now know that you want/need to have the bottom ends of the top legs to match up and sit right on top of the tops of the bottom legs. When you glue the top section of the tower to the bottom section, you end up with one-piece (because they’re glued together) legs. If you look at just the bottom section, as we discussed above, the ladders joining the tops of the legs are under some compression load- because of the downward load, and because they’re angled in toward the centreline, the ends of the legs are “trying” to collapse inward toward the center, and the ladders are resisting that force, working to hold them out in place. The bottom ends of the legs of the top section are, because they are at a bit of angle, “trying” to push outwards. But, because they are glued to the top of the bottom legs, they can’t (with a decent glue joint) go anywhere; they’re good to go. The way ladders work in a bracing system is they resist the legs buckling inward toward the center, and toward adjoining legs. They work under some level of compression force - the legs are pushing against them; the diagonal bracing works to keep the legs from moving outwar - outward from the center, outward from each other. So, at the bottom of the upper legs, there is no need for a ladder to brace them from moving inward- load forces are pushing them the opposite way- that "second crossing bar" is jusr excess weight.

Hope this makes a few things clearer, gives you some new things to think about, and takes some of the mystery out of things; best of luck, this year!

[illusionist]

I think this wins the "Longest Post of the Year" award... O.o
(I was going to quote it, but i don't think the site mods would have been too happy with me =P)

[Costarica]

What is considered a good score this year? This would probably just be an opinion

[deezee]

What is considered a good score this year? This would probably just be an opinion

Well technically you want the lightest tower that holds 15 kg and is 70 cm tall.