Building a Wooden Tower

[evolllracer]

Hey does anyone know a good tower design using balsa wood?
my requirements are 2" by 2" (thats the base) and the height is 8"
and has to be less than 20 grams
ive tried soaking the wood so that it could bend into a circle because that was the design i had in mind but that didnt work
please help my tower is due in 2 days

[Buffy]

Is there any goal other than it be less than 20 grams?

I'd say two intersecting 2" pieces for the base and a single pole extending up from the intersection is the lightest you're going to get and have it stand up. You might save some weight by making one or both of the base pieces only 1" if having a 2" dimension in both directions isn't a requirement.

Good luck!
Buffy

[evolllracer]

oh sorry i wasnt specific enough
50% of the grade would be however much weight the strongest tower holds so if the strongest holds 100 lbs and my tower holds 10 lbs. then thatd be 10% of 100 so id end up with only 10 points
so it has to be under 20 grams and has to be strong

and thanks for the help guys

[InfiniteNow]

I'm no engineer, but I would think a design like the following would be pretty strong:

[GAHD]

easy, in text, these are your walls:

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the angled peice braces it, only one top to bottom brace sould be necessary with a strong enough bond at the corners.

balsa is flexible when moist, keep it moist to allow it to flex before it breaks, allowing you extra weight there.

3 walls is the strongest structure you'll get to stay standing, the crossbrace when corkscrewed adds a bit of stiffening without totally destroying flexibility. cut your peices to height and length, then shave 'em down to weight, but keep them even lad!.

hope it helps, let us know how it turned out, and bring pictures!

[CraigD]

First, lets gather some specifications on the required material, balsa wood. The compression strength of balsa in the direction of its grain is about (2750000 N/m^2 (400 psi) (according to test results from Balsawood Structure Design). The density of dry balsa wood is from 100 to 200 kg/m^3 (source: wikipedia article “Balsa”[/url]). For this discussion, I’ll use the worst case density of 200 kg/m^3, or .2 g/cm^3.

There are 2 basic ways to approach this problem: begin with your maximum allowed mass, and design a structure that can support the greatest weight, or begin with the target weight, and design a structure with the lowest mass that will support it. I’ll follow the first approach.

Your tower must mass less than 20 g. Mass / density = volume, so its max volume (of material) is 20 g / (.2 g/cm^3) = 100 cm^3. It must be be 21 cm (8”) tall, so its maximum cross-section is about 4.75 cm^3. Force = pressure * area, so the maximum weight it can bear is about 1300 Newtons (292 pounds).

So, the design target of supporting 100 pounds seems achievable.

The structure has to do more than withstand a vertical compression force – it must withstand lateral and torsional (tipping and twisting) force. Lots of possibilities exist that can do this – the simplest, IMHO, a rectangular box 5 cm by 5 cm by 21 cm tall, closed at top and bottom (inside, not on top of, to allow the balsa to bear the weight in the direction of its grain of its walls, not crushing against the grain of the top and bottom pieces).

Normally, I’d leave the calculations to evolllracer, but as the assignment was due 3 days ago, will do them myself. Keeping the 6 pieces the same thickness, their total area is (21*5)*4 +(5*5)*2 = 470 cm^2, with a thickness of about 2 to 4 mm, depending on the actual density of the balsa. Its weight-bearing cross section will be 5 to 10 cm^2, so it should be able to support between at least 1375 and 2750 N (309 to 618 pounds).

I’m curious to know how the various balsa tower projects turned out – this one, and the one in the older ”How to build a wood tower”.

[Turtle]

Originally Posted by CraigD

...
I’m curious to know how the various balsa tower projects turned out – this one, and the one in the older ”How to build a wood tower”.

Ditto Craig. I am curious as well about the lecture and course material content that precedes these assignments. What information is given on structural integrity?

I recall watching similar contests on TV wherin the towers are subjected to more & more weight until they collpase and whichever tower held the most before the collapse is declared the winner. They never show follow-up discussions, so it's hard to know if they have them; seems to me such a discussion concerning why a particular design fails at a particular time is an essential part of these experiments.

I am tempted, yet hesitant, to submit an actual design. The whole 'doing your own work' bit keeps naggling me.