Towers B/C

[bernard]

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[DarthBuilder]

Can someone help me understand the new rules for towers (Div C?) I was told about the bonus change, but are there any other really big changes? like two pieced? thanks,

DB

[cool hand luke]

Bonus is up to 5 KG, for division c at 20 cm you have to be under 8 cm.

[Balsa Man]

OK, with the new rules officially out, let’s go over the differences and what they mean.
Let’s start with what didn’t change:

Height- still (a minimum of) 50cm for B-Div, 60cm for C-Div. Means, as last year, you have to be a bit more precise, getting everything lined up symmetrically in C-Div, and that C towers will be a bit heavier than B towers.

Load block/top dimensions- still has to support a 5cm square load block on the top of the tower. This means, as last year, you want the tops of your legs positioned so they fully fit inside of/underneath a 5cm square; you don’t want the load block just partially covering the leg ends- you want the load to be supported by the full cross section of the leg ends.

Base (footprint) dimensions-still one of two basic options; first is clearing/spanning a 20cm x 20cm square opening in the test base, second is clearing/spanning a 29cm diameter circle (centered around the 20x20cm square). Looking at the spanning 20x20 square option- this does not mean your base leg positions have to be a square that will sit outside the corners (i.e., a 20+ by 20+ square); if you draw it up, putting the leg ends at the middle of each of the 4 sides of the 20x20 square, you’ll be looking at a square base approximately 16cm x 16cm. The second option, spanning the 29cm circle, you’ll find when drawing that up that with a square base footprint, you’re looking at a square of approximately 21cm.
There is a good reason (discussed in a minute) to span the 29cm circle- the “circle bonus”, where you get extra credit for load carried by the tower if it spans the 29cm circle.

There’s a… sub-option to consider if you go for spanning the circle; a rectangular base footprint. A square base will need adjacent legs separated by a bit over 21cm. If you go to a rectangle, you can have two (opposite) sides much shorter than 21cm, for instance around 8cm, which would mean the other opposite sides would be around 25cm. You might think because the bracing pieces between legs on the “short sides” are so much shorter, you can save tower weight with a rectangular base. My experience is this is not the best way to go- the weight penalty for the much longer braces on the ‘long sides’ takes away any advantage. Also, every year, the idea that a 3 leg tower (triangular base footprint) can be lighter than a 4 leg tower comes up. There are a number of factors that work together to say you cannot be as competitive with a 3 leg approach.

Remember to think about clearances; for both options, the inside edges/corners of the legs have to clear- as in be outside of either the square or the circle. Because the more the legs are angled/sloped inward, the more force they have to carry (at a given load), you don’t want any more clearance of the circle or square than you… need. You want the base ends of the legs to clear the square/circle by just enough to be sure you meet the rule, to be sure that the judges checking you in for competition will confirm you comply with the rules. If you build with good precision, providing 2mm clearance on all sides will work. Less than that, you’re asking for trouble. Going to 3mm all the way around is conservative; certainly no need for any more than that. This guidance applies for height, too, meaning 60.2cm height for C, 50.2cm height for B.

And then to the changes….

Clearing 8cm circle at given height above the base- Big change. Last year, you only had to clear base opening/circle at the bottom, and support 5cm square load block at the top. That meant straight, one piece legs; towers that were truncated pyramids. The change brings back the same approach used in the 2011/2012 tower rules. For B-Div, everything more than 25cm above the test base surface has to fit inside an 8cm diameter circle; for C-Div, everything more than 20cm above the test base surface has to fit inside an 8cm circle. This means towers will need to be in a “two part” configuration- the base, or lower part, with fairly steeply angled legs, and, sitting on top of it, the upper, or chimney part with minimally tapered (near vertical) legs.

The need for designing in clearance to be sure you meet the rules, and that judges will be sure to classify you as meeting the rules is important. The typical way for measuring compliance is to take a thin sheet of….something (plexiglass, aluminum plate, posterboard), cut a circle 8cm in diameter in the middle of it, make some support legs 20cm (for C towers) and 25cm (for B towers) long. Then lower the plate with the circle cut out over the top of the tower, and take it down until it (hopefully) sits on the support legs. If it hangs up on the tower before it gets down to the support legs- i.e., the tower does not fit within the 8cm diameter circle at 20/25cm above the base, that’s a construction violation- you don’t meet the rules; you’re moved down to Tier 2; you score behind all the towers that did meet the rule.

It will take some care/work to create a precise tool, so you don’t want to build your towers to just barely meet the dimensional spec, and count on precise measuring at competition. Again, a 2mm clearance is, I think, a reasonable minimum to work with/design to. So, the legs supporting your “test hole” plate (and you really need to build your own to be able to confirm your builds meet the rule) should be 18.8/24.8cm long. The height at the top of your base section should be no more than 18.8cm/24.8cm. The diameter of your “8cm” test hole should also provide some ‘safety clearance’, i.e., should be a bit smaller than 8cm, because you can’t be certain that the judges will have an absolutely precise measuring device to check rule compliance. By providing the 2mm height safety margin noted above, you can probably be safe using a 7.8cm test hole (which will give you a 1mm clearance all the way around). You could choose to make the test hole a bit smaller; I would not recommend going any larger.

You could conceivably do your base section so that the top of legs configuration fits under the 5cm square of the load block, and your upper/chimney section therefore runs vertical legs (saving some bracing length/weight), and has plenty clearance inside an 8cm circle. I personally don’t think that tradeoff is worth it, because the price is the tower becomes… much more sensitive to load sway/movement. If there’s a bit of angle in the chimney legs, you can have a bit of load bucket swing before the load in one leg (the leg the bucket swings toward) really starts to go up. You want the leg strength to be just barely enough to carry full load, when its evenly distributed between the 4 legs; if one leg gets disproportionally loaded, it will fail first. With the bottom of the chimney segment legs fitting inside an 8cm diameter circle, and the top of them fitting inside a 5cm square, you can’t get much lean-in, but what you can get helps. You can pick up some by closing in the top a bit, say fitting a 4.5cm square instead of a 5cm one. Problem is, it’ll be more difficult to get the load block centered on the top, and if its off just a bit, you’ll be disproportionally loading one (or 2) legs….

29cm circle bonus weight credit- this is a significant change. Last year, you got a 2kg credit added to your weight carried (e.g., carry 14.5kg, get credited with 16.5kg to calculate score). Under this year’s rule, you get a 5kg credit. So what does that mean? To get the base section legs spread out/angled enough to sit outside a 29cm circle (compared to a base spanning the 20x20cm test base opening), they’re going to have to be longer, and they’re going to have to be stronger; and both those factors mean more weight. Weight for bracing will also be higher, because down toward the bottom, the span between adjacent legs is longer (about 16cm at the bottom of a non-circle bonus tower; about 21cm at the bottom of a circle bonus tower). Last year, it was close- whether a circle bonus, or non-circle bonus approach was more efficient. The increase tips the scales, so to speak.

Let’s look at a 6gr tower (non-circle bonus) example, carrying full load. Scores 15000/6.0, which = 2500 score. Under last year’s rules, you could add 0.8gr (for a tower weight of 6.8gr), and still score 2500 (17000/6.8). Under this year’s rules, you could add a full 2gr, and still score 2500 (20000/8.0). I’m going to do some more calcs to make sure, but I believe it’s a no-brainer.

So, this is all saying the competitive approach is going to be a “2-piece” design, with the bottom section meeting the 29cm circle bonus. What else is important to think about?

Experience back in 2011/2012 says that building separate bottom/base section, and upper/chimney section, and setting/glueing the chimney section on the base to make a complete tower, is a real bear to get right- getting all 4 upper leg ends to sit perfectly on top of all 4 base leg ends is really hard to pull off, and getting things aligned so that the chimney sits perfectly vertical adds a significant degree of difficulty. If you don’t get both aspects of this alignment really, really close, leg load is not equally distributed, and overloaded leg fails prematurely. I believe that while it has its own challenges, a “one piece” jig is the way to go.

A jig, for the newbys, is a structure you build that holds the legs in position, in good, symmetrical alignment, and allows you to put on bracing while keeping that ‘perfect’ alignment. There are a number of ways to construct a jig that will work, and we’ll be discussing in more detail soon. The key, I believe, is a single jig that holds both bottom and top leg segments of each leg in place.

The last significant… implication of the new rules is in wood selection and bracing. The lean-in angle of the base segment legs is going to be significantly higher than the lean-in angle of last year’s straight legs. That means that the force on the lower legs (for any given tower load) is going to be higher than last year, and that means the leg wood in the lower legs will need to be stronger (i.e., heavier leg wood and/or heavier bracing), and conversely, the upper leg segments will have less lean-in, meaning lower force than last year (lighter leg wood and/or lighter bracing).

Because of the lean-in of the base segment legs, there is going to be significant force pushing the tops of the lower legs together (on the order of 2kg for a C tower). That means a ladder brace at the top of the base section, between each of the legs. We’ll be getting into more discussion, soon, on how to approach bracing, but for a number of reasons, I believe the optimal approach is “all Xs” (as opposed to “ladders and Xs”), because a) last year’s results showed an all Xs system is more efficient than ladders and Xs, and b) in the nearly vertical chimney section, it is extremely difficult to build a jig that will allow the chimney portion to be lifted vertically off the jig after the ladders are glued in-place between the legs.

So, that’s my initial run-down on the implications of the new rules vs. last year’s. Looking forward to good discussions; I’ll be getting out more detailed information on how to approach figuring out the design and construction details that will get you to a competitive tower.

[DarthBuilder]

How many inches do you think the chimney should be?

[bernard]

How many inches do you think the chimney should be?

I would make the "chimney" around the minimum height required for that section.

[DarthBuilder]

There isn't a minimum or maximum height for the chimney. how high do you think the tower should be? I might've read wrong though but I'm pretty sure no requirement of how tall it can be. Except that after 20 cm part.

[Unome]

There isn't a minimum or maximum height for the chimney. how high do you think the tower should be? I might've read wrong though but I'm pretty sure no requirement of how tall it can be. Except that after 20 cm part.

The rules specify a minimum tower height of 60 cm, and the 8 cm ring must extend down to at least 20 cm from the Test Base - this leaves 40 cm in between. I would go for a 40.4 cm chimney, to account for variables (shrinkage of wood due to humidity changes, differences in measuring device of the event supervisor, etc.).

[cool hand luke]

Balsa
In last years discussion you posted the leg buckling vs weight, on that graph you had multiplied it by 2.0 and 2.3 fe's based on fixed fixed end conditions


You then stated the all x design with no ladders seemed to have a fe of .4.


Did you mean this is the reciprocal (the effective length? I can't make it jive with the 2 and 2.3
But a .4 as effective length and a 2.5 fe would make sense.

[Random Human]

Anyway to build a one piece jig balsa? Sorry storming you with questions.

Random