Designs

[SLM]

What is the thing in the middle made of? THANK YOU!

We used plywood for the base and balsa for the vertical part, both pieces bought from an arts and crafts store.

[iYOA]

is there anywhere you can get them to cut the pieces a specific dimensions?

[fishman100]

is there anywhere you can get them to cut the pieces a specific dimensions?

A lot of online companies allow you to customize your oder, so you can get a piece of wood for pretty much any dimension you want. Also, I think Home Depot has a wood cutting service (first 2 cuts are free).

[iYOA]

we tried home depot but dont really custom cut that much. like it can only be a straight cut(not angled) and they dont cut along the length

[SLM]

we tried home depot but dont really custom cut that much. like it can only be a straight cut(not angled) and they dont cut along the length

You are right; Home Depot is not the right place for cutting wood for this project. We did not have to cut any of the pieces for our jig. Most arts and crafts, or hobby, stores sell small rectangular pieces of plywood suitable for this purpose. If necessary, you can cut the balsa piece using a hand saw.

[Cheesy Pie]

Try a square tower base, and a lot of triangles in the tower itself. Triangles are sturdier than squares.

[mrsteven]

I have a question, in terms of best to hold weight/ what you guys do, where do you position the cross braces on your tower? Ei like inside the main structure, outside, in the center? And where do you put the ones that cross them?

Soo after ranting I guess my question is if you put on set of braces in between the main supports and the ones that cross those outside/inside / whatever . . .

[Balsa Man]

I have a question, in terms of best to hold weight/ what you guys do, where do you position the cross braces on your tower? Ei like inside the main structure, outside, in the center? And where do you put the ones that cross them?

Soo after ranting I guess my question is if you put on set of braces in between the main supports and the ones that cross those outside/inside / whatever . . .

What the ladder bracing does – the ‘cross pieces’ – is prevent the legs from bowing in toward each other. They are much more efficient at doing that- taking the compression load that puts on- if they’re in between the legs –ends of ladders in a butt joint against sides of legs. The ‘ones that cross them’ – as in run diagonally between where the ladders join the legs work in tension, acting to prevent the legs from bowing out away from each other. Lap-joint on the outsides of the legs is most efficient for them. I’m sure we’ll hear other thoughts on that, just speaking from what’s worked for us in years past…

[mrsteven]

As I am new to the towers designing, what do you mean by "butt joint against sides of leg"? And Lap joint?

You say ladder bracing- meaning the braces are parallel with the ground? I see most towers with X's for the bracing. So am I misinterpreting something O.o

[Balsa Man]

As I am new to the towers designing, what do you mean by "butt joint against sides of leg"? And Lap joint?

You say ladder bracing- meaning the braces are parallel with the ground? I see most towers with X's for the bracing. So am I misinterpreting something O.o

FIRST:
As I and many others have said many times before - and it applies to this and many other questions that keep popping up - go back, through this year's posts, through the archived posts from last year, use the search function. There is an amazing amount of useful information that's sitting there waiting for you to look at it.

One relevent example - from the "Bracing" thread from last year:
Re: Bracing
by Balsa Man on Tue Mar 01, 2011 11:54 am

Haven’t had a chance to be on the board since Christmas (way busy at work).
As I’ve said before, there are many ways to skin a cat (and do bracing). A little review of the way we’ve chosen to do it (dig back to posts last fall discussing this):
We use (horizontal) ladders and X-bracing. Tower legs are 3/32nds. Ladders are also 3/32nds. Xs are 1/64th x 1/16th to 3/32nd. You can see what it looks like by looking at bridge pictures from 2009 – along the top of the bridge, joining the two top members. In a tower, those top members would be the legs.
Here’s why and how it works. SLM is pretty much right-on on the function of the bracing system- prevent buckling of the legs (the intervals you put ladders at divide the legs up into shorter exposed column length-shorter column lengths can take higher load before buckling) – and keeping legs “upright and aligned.” One important function beyond that; together, the bracing system makes the entire structure (tower) rigid.
Let’s visualize. Looking at just one side of the tower, and one “section”- three ladders- one at top, one in the middle, one at the bottom; laying on a flat surface. You may even want to cut a few pieces of wood and try/feel this. Between any two ladders, you have a trapezoid- parallel top and bottom, with the two sides somewhere between vertical and, oh, maybe 20 degrees sloped in on the base section.
If you “pin down” the four corners (i.e., if they can’t move), and push the middle ladder horizontally, what happens? It moves horizontally, and the legs bow. What happens if you join the corners where the ladders meet the legs with Xs? Let’s start with a brace from the lower left to the middle right, and from the upper left to the middle right. With them in place, what happens when you try to push the middle ladder to the right? Those brace pieces come under tension, and they keep the ladder from moving right. They would do exactly the same if they were string, instead of wood; the only way for the ladder to move right would be for the braces to stretch (lengthen); if they don’t stretch, the ladder doesn’t move. Now, let’s try moving the middle ladder to the left. What happens? It moves; the half-Xs bend (the points they’re attached to move closer together, shortening the distance between them). OK, now put the rest of Xs in place; pieces from the lower right corner to the middle left, and from the upper right to the middle left. Now, which ever way (left or right) you push the middle ladder, it can’t move.
If the legs try to bow inward, the ladder (acting in compression) resists that movement. If the legs try to bow out, the Xs (acting in tension) resist that movement. If now add another side (at 90 degrees in a 4-legged tower, at 60 degrees in a 3-legger), this same bracing works in 3 dimensions. Put all four (or 3) sides together, and you have a rigid structure.
The important thing to understand here is that the ladders work in compression, and the Xs in tension. The Xs (if the rules allowed) could be fine thread. The key is that they be tight. If they have any slack in them, then where they join has to move before they tighten up. That movement – buckling starting in the leg, starts the failure process. If the Xs are tight, and where they join can’t move, then that point of the leg stays in place, and everything is cool. In the upper portion of the tower, where the legs are near vertical, it doesn’t take much strength to hold the point where the Xs meet in place. Down in the base, the Xs going to the lower ends of the legs will be under significant tension load (the bottom ends of the legs trying to spread out). That force is on the order of a few kg. High density 1/64th balsa is amazingly strong in tension; pieces 1/16th wide can hold 2-3 kg (try it). 1/16th provides plenty of glue area (joint will be stronger than the wood).
So, for what its worth; it is one way that works.
There is additional discussion in the Design thread (see pg 17)- It would be well worth your/anyone's time to read through last year's discussions. In the bracing thread, there is some great detail on bracing systems from SLM. As you will see from the disxussion, there is not a single "correct answer" to the "best way" to do bracing' there are various ways that have been successfully used by folk. When you say you see most towers with X-bracing, I would agree, and a lot of that is with Xs and ladders. To do column bracing, it takes both horizontal and diagonal. There has been, and will be going into the future, a lot of discussion on Xs vs Zs (double diagonal vs single diagonal). The efficiency - the weight penalty you pay, and the effectiveness in preventing buckling failure, AND in putting structural stiffness into the structure depends on what wood you use. Our experience is that the system I've described above is very effective.

SECOND
A butt joint is one where the end of one piece-one stick "butts" against the side of another. Specifically for this discussion, where a "ladder" (which is a horizontal - as in parallel to the ground) bracing piece that runs between two adjacent legs- the legs are like the sides of a ladder, the "ladder piece" is one of the cross pieces you step on. A lap joint is where one piece "laps" over another. If you lay two leg pieces side-by-side, and instead of putting the ladder piece in between them- in the same plane as them, you have it a bit longer and lay it across- on top of them- then where it joins them is a lap joint. Lap joints work well in tension; butt joints work well in compression.

[LKN]

Balsa Man

How did you get ahold of 1/64 x 1/16 inch balsa? Specialized Balsa doesn't offer those strips, but they do have 1/64 inch sheets. Did you buy 1/64 inch sheets and cut them? On that note, what is the most precise way to cut thin balsa sheets? I usually use a ruler and a razor blade, but I never seem to be exact as I want to be and end up abandoning the idea to use hand-cut balsa strips for my X-bracing for a chimney. Because of this I have been using 1/16 the lowest square balsa (much cheaper than online, but takes time to individually weigh each stick) at my local hobby store.

[Balsa Man]

Balsa Man

How did you get ahold of 1/64 x 1/16 inch balsa? Specialized Balsa doesn't offer those strips, but they do have 1/64 inch sheets. Did you buy 1/64 inch sheets and cut them? On that note, what is the most precise way to cut thin balsa sheets? I usually use a ruler and a razor blade, but I never seem to be exact as I want to be and end up abandoning the idea to use hand-cut balsa strips for my X-bracing for a chimney. Because of this I have been using 1/16 the lowest square balsa (much cheaper than online, but takes time to individually weigh each stick) at my local hobby store.

Yup, cut from 1/64th sheet. Done with a razor blade and ruler. A few tips on doing that:
Don't try to cut them too long; cut the sheet down into sections a bit longer than you need the strips to be, and strip from them.
Get a ruler with ....a nice hard edge. We use one from a square - one of those squares with a 90 degree & 45 degree angle, cast metal piece with a slot in the bottom that the ruler fits into/slides along. The ruler is steel, and the edges are hard/sharp. When you press it down on a piece of sheet, it doesn't slip around.
Use a surface to do the cutting on that doesn't let things slide around too easily- a plastic cutting board works well.
Use a "guide piece" to line up the ruler; cutting 1/16th wide strips, use a little piece of 1/16th square; work at both ends of the ruler/sheet, putting the guide piece down, aligned with the exposed edge of the sheet, move it back and forth, adjusting the ruler till you have it lined up- parallel to, and a 16th back from the sheet edge.
Use two fingers spread well apart to get the ruler firmly held in-place.
Don't try to make the cut in one pass- make the first couple passes with very light blade pressure, then gradually increase pressure with each pass. With high-density wood, the grain structure can be quite solid, and will tend to deflect the blade off a straight cut line.
Use a sharp blade; buy a big box of blades; we actually sharpen from out-of-the box (w/ a really fine sharpening stone)
Don't expect 100% success; of the strips you cut, most should be keepers, but some go to the scrap pile
Before you strip, look at your sheet carefully; hold it up to the light; mark areas (w/ a magic marker/highlighter) that are.....suspect- visible holes, areas that are much more transparent when held up to a light; areas where you see significant diagonal grain structure. Don't use strips that have marked areas in the middle of them.

That's pretty much the set of tricks for cutting useable strips; have fun!

[silverheart7]

Do you think a tower is better with a longer chimmney or a shorter one?

[Balsa Man]

Do you think a tower is better with a longer chimmney or a shorter one?

Well, that depends on what you mean by better....
The way the scoring is set up, taller is better - you get a significant scoring bonus for going from the minimum of 40cm up to the max of 70cm - and that bonus, best I can calculate, is bigger than the cost in weight/efficiency that building taller costs you.
The rules define where the base stops, and the chimney begins - where the tower has to fit inside the 8cm circle. The base is up to 30 cm (in a B-tower); the chimney is the skinny part from there up. So, to get a taller tower means a longer chimney, and circling back to the start, a longer chimney/taller tower scores more points.

Now, that all assumes that the taller and shorter towers both hold the same load. If because you don't/can't build with enough precision to get a 70cm tower to hold full weight, or about the same weight as you could hold with a shorter tower, you'll be better off with a shorter one.

Does this answer your question?

[baker]

For those that were calculating downward load forces... If the chimney had a 4 cm square top and a 5 cm square bottom (where it mates with the base), how much of a load change is there on the mains? Is it better to remain vertical or have a little angle? Thanks..