How to build a balsa wood tower

[hippo9]

I'm assuming this doesn't have to span a center gap like scioly towers right?

[knightmoves]

So you actually have quite a lot of information. You are given 4 pieces of 1/8 inch square by 24 inches balsa. Presumably you have a sharp x-acto knife to cut it, and some glue (most people would use cyanoacrylate glue (superglue)). Your tower must be at least 8 inches tall.

(You say that the tower has to weigh less than 20g. A tower made from 4 24" sticks of 1/8" balsa will weigh less than 10g, even with dense balsa, and might weigh less than 3g. If you get to choose the balsa, choose heavier sticks. At 24" length, it could easily be hobby store bass rather than balsa, of course. You still do the same thing.)

First, go and look up the formula for Euler's critical load. Wikipedia has a good explanation. You see that shorter columns hold more weight, so you're going to want to build your tower exactly 8 inches (plus a bit, because your ruler might not be the same as your teacher's ruler. So maybe build 8.25 inches.

You want 4 legs, because your legs are square, and you're going to need to join them together. So that uses up 2 17 inch lengths of your wood. (Cut them a bit long and sand them square and equal. You want your tower to be symmetric and well-balanced.)

So you have remaining 2 24" lengths and 2 7" lengths with which to connect your 4 legs. A wider tower is more stable, but you'll run out of wood fast. Go read the Euler's critical load section again, and read about bracing in this forum. You make a stronger tower by shortening the effective length of the legs by joining them to each other at regular intervals.

There are lots of different bracing patterns you can use. Everything will make triangles (because you can't shear a triangle), but your braces could look like a repeated X, Z, or similar as you look at the side of your tower.

1/8" balsa is quite thick, so if you have braces crossing each other (X pattern), you'll want one on the inside of the legs and one on the outside, so you're not trying to bend the braces. Z pattern would be easier to build.

You have 2 24" lengths and 2 7" lengths left, and need to make 4 tower sides, so that's 15" of balsa per side. Imagine you want to make 2 Zs down the side of your tower out of that 15", and you end up with legs about 2.5" apart - a little under, really. Maybe have the top 1/4" smaller than the bottom, so the tower has just a little taper, to give it some stability. If you taper, you'll want to sand the tops and bottoms flat at the end, so the tower isn't standing on the corners of its legs.

Plan all your cuts before you make any, as the key here is to use all your balsa efficiently. If you mess up and are one brace short, your tower will be very bad.

Draw a side of your tower on a sheet of graph paper, and use it to check your cut lengths. Also assemble the tower sides on the drawing, so you get everything put together square. This is important - if you build it unevenly, all the weight will end up on one leg rather than 4.