Towers B/C

[Gr8tor]

Balsa Man, will it be a problem if I cut the edge strips into 2 or 3 parts and glue them all together afterwards on the acrylic. The 3D printer I'm thinking of using has a bed the size of a 15 cm square. Are there places out there where they can 3d print things for you. How accurate are 3D printers?

[Balsa Man]

What bracing intervals would work best for a division B tower this year?

Yes, you have summed up the entire tower competition in one sentence!
The bracing interval is a function of the column density and stiffness. As the column density goes up, the number of bracing tiers can come down (theoretically, anyhow). The lighter the columns, the more bracing will be required.

First, just for clarity, when you say “work best”, I’m assuming you mean will get you the highest scoring tower. And yup, figuring out what will work best is the problem; the challenge; what this game is all about.

Second, dholdgreve is describing the fundamental tradeoff you have to understand and figure out to answer the question of what will work best. It is a tradeoff, because with stiffer, heavier, stronger leg wood, the bracing interval that will work (where ‘work’ means the leg segment has been braced enough to carry your design load) can be longer, more open, meaning you’ll have less wood weight in your braces. Conversely, with floppier, lighter, weaker leg wood, the bracing interval that will work has to be tighter, shorter, meaning you’ll have more wood weight in your bracing. The type of bracing also affects the bracing interval that will work with legs of various strength.

I’m not going to just give you the answer (or what I think the answer is) to your question, but I’m more than glad to share some basic information, and a couple of engineering tools that will give you what you need to work out the answer your question. It’s like the difference between giving someone a fish, and teaching them how to catch a fish. What you need to be able to figure out, and look at, and compare is the weight of the leg wood + the weight of the bracing wood + the weight of glue needed, for a range of bracing intervals. With this year’s ‘2-part’ rules, you actually need to look at this tradeoff separately for the upper/chimney section, and the lower/base section.

For each section, this is a 2-part problem. The first problem is a strength problem; the second problem is a density problem.

Looking at the strength problem first, when I say strength, I’m talking about buckling strength. “Stiffness” is… another way of saying this. How much force does it take for a braced segment of a leg to start to bow out in the middle (which is what buckling is)? - stiffer means has higher buckling strength. Buckling is how axially loaded thin columns fail; the legs in a tower are axially loaded thin columns. Once buckling/bowing starts, its all over. With the same force that started the bowing of a leg segment being applied, the bowing will progress… very rapidly, and the segment will snap.
We know buckling strength has an “inverse square relationship” to length. What is that saying? When you brace a column (a leg section), you create a set of shorter ‘stacked columns.’ If a full leg segment (be it chimney or base segment) at length L has a buckling of X, if you brace it at the middle, you have two shorter columns/braced intervals with a length of L/2 (the proportion of the braced intervals is ½), and a buckling strength (BS) of 4X. If you brace it into thirds (a 1/3 bracing interval – a proportion of 1/3), the three braced intervals will have a buckling strength of 9X, and so on. The general form we see here is that the BS of a shorter braced interval is 1 over, as in 1 divided by, the proportion squared; that an inverse square relationship. “Proportion” is the proportion of the shorter interval to the longer interval.

As discussed many times, we can measure the BS of a stick (like a 36” stick), and do an inverse square calculation to know what the BS will be increased to at a given bracing interval. One could do a bunch of inverse square calculations, looking at a range of stick buckling strengths, and bracing intervals, to see what bracing interval would be needed, for what stick buckling strength, to get a leg strength that would work- as in carry the force the leg will see when the tower is loaded. A spreadsheet with a range of braced intervals, and a range of measured stick BSs, calculating the braced BS for a given bracing interval, and given stick BS is the best tool for doing this. With it, you can solve the strength problem- to know what strength wood you would need, to have various bracing intervals “work.”

On to the density problem; with a way to know what strength leg wood you will need to use each of a range of bracing intervals, we now need a way to figure out what weight/density of wood it will take to have, to get, a given BS. If one had data on a whole bunch of balsa sticks – for each stick, the stick weight, and the measured BS, and you put that into a graph plotting stick weight vs BS…..what could one do? You could look in your inverse square table and, for each of the bracing intervals you’re evaluating, see what stick BS you need, and look at the graph of stick weight vs BS and see the stick weight you would need to have the BS you need. This sort of process, and development and use of tools is at the heart of ‘”engineering,” and “the design process.”

These two basic tools you will give you what need to evaluate the tradeoff we started this discussion with. With stick weight, you’ll be able to calculate the leg weight (that would work) for each bracing interval. And, you can easily figure out the length of the bracing pieces needed for each bracing interval (by just measuring them off a scale drawing)- then you’ll know how many total cm of bracing wood needed for each bracing interval being evaluated.

Then, with bracing wood length, you can calculate bracing wood weight (by using a density value and cross sectional size for the bracing). Actually, you’d want to use two or three values; a light, medium, and high, and calculate the grams per cm of bracing wood for each of the three scenarios. Gr/cm times total bracing length (for each of the bracing interval scenarios) gives you weight of bracing wood. If you’re using an “all Xs” bracing approach, with 1/32” x 1/16” (‘the hot setup’ from last year. And no reason to believe it won’t be this year), light would be about 6gr 1/32” x 3” x 36” sheet, medium about 7.5gr sheet, and heavy about 9gr). Last factor is glue weight; you need a little bit of glue at each end of each bracing piece. A workable number for the evaluation we’re trying to do is 0.03gr/joint.

So, what are you going to have to do to get the two basic tools I described?.

Go back to page 6 of this thread. See the links in my long my post on Oct 20, near the top of page 6. Read the post, click on the links, study, and enjoy.

The inverse square table is set up for all Xs using 1/32 x 1/16 Xs. If you want to look at ladders and Xs, you need to change the effective length factor in cell N8 to 2.3 (and add lines to look at wider bracing intervals). The post discusses what the effective length factor is all about.

[Balsa Man]

Balsa Man, will it be a problem if I cut the edge strips into 2 or 3 parts and glue them all together afterwards on the acrylic. The 3D printer I'm thinking of using has a bed the size of a 15 cm square. Are there places out there where they can 3d print things for you. How accurate are 3D printers?

Sorry, I don't follow what you're thinking about on the cutting/glueing
Yes, there are many on-line services that do 3-d printing (and laser cutting)
How accurate, depends on the printer; typical school/home use ones are plenty accurate; better than 1/10mm
We print the edge pieces vertically in the printer, 5cm long. Then just glue along the jig plate edges.

[Gr8tor]

How much does grain matter in towers. I think C grain since it's stiffer but, I'm not sure.
It would be extremely helpful if you can give an intro to the science behind getting the best possible balsa. I've heard of stick testing in this form but I'm not sure what that is.

[Balsa Man]

How much does grain matter in towers. I think C grain since it's stiffer but, I'm not sure.
It would be extremely helpful if you can give an intro to the science behind getting the best possible balsa. I've heard of stick testing in this form but I'm not sure what that is.

On getting 'best possible balsa', more than glad to provide such information. I already have posted detailed discussion, multiple times, many pages, this season and last season. Most recent was just three posts above this one (specifically addressing "how to get the best wood"). To get an intro, and a lot more, to the underlying science, and detailed discussion of exactly how to use that science, all you have to do is be willing to read and work/study understand what is already here. That's up to you; I can't help or make you do that. What can I say......

On grain, have also discussed this a number of times; A, B, and C grain is something that applies to sheets. It comes from the orientation within a balsa log that a sheet was cut at. Square sticks do not have a grain pattern- two opposite faces will have one, and the other two opposite faces will have another.

So, you have some homework to do, but the textbook and study notes are sitting at your fingertips waiting for you.

[Gr8tor]

I've got a question
I have 2 pieces of wood with the following properties

i. They are the same length

ii. They have the same weight

iii. One has a larger cross section than the other

Which one is stronger?

[Balsa Man]

I've got a question
I have 2 pieces of wood with the following properties

i. They are the same length

ii. They have the same weight

iii. One has a larger cross section than the other

Which one is stronger?

Ah, you haven't done your homework yet.....

Error message; insufficient data.....

Assume you mean which has higher buckling strength, yes?
Without weight and cross section, you can't calculate relative buckling strengths
So, what are those data, and what do you think the answer is?

[Raleway]

I've got a question
I have 2 pieces of wood with the following properties

i. They are the same length

ii. They have the same weight

iii. One has a larger cross section than the other

Which one is stronger?

F = pi^2*E*I/(KL)^2

Do some research like Balsaman said- we have so SO much information on this forum. This question has been discussed probably over the length of many forum pages (or topic). The formula listed is Euler's Critical Load.

*EDIT* You also have not defined which direction the force applied is, what type of force, and a whole set of data types yet to be listed like Balsaman said. Also, make sure you know what "strong" means- it means resists both deformation and failure.

[Gr8tor]

Ok, I though the answer would be more simple than that. It was more of a general question. I don't have any specific weights or cross sections to give you.

[Gr8tor]

I know its been a while. For the solvent you (Balsa Man) showed me.

https://www.amazon.com/dp/B00JFPF0UQ/re ... ullets-btf.

Does it also bond with the edge strips. I'm thinking of using PLA or ABS plastic for the edge strips.