Designs

[Paradox21]

I have been designing a triangular tower, with a triangular chimney. However, I found that in order to make the top of my chimney small enough to fit the loading block, the inner circle that accomodates the eyebolt became too small (it needs to be 3cm in diameter). As shown here:

The diameter in this figure would be 2.82cm.
Has anyone designed a triangular chimney that fits the eyebolt?

[rjm]

Let me try to clear up the interior opening question. This is my interpretation, which I believe to be correct. For official clarifications, you must go through proper channels.

The requirement that the eyebolt fit through a 3.0 cm opening is in 4.d., Testing Apparatus. It describes the maximum size of the head of the eyebolt. It is a limitation on what the ES is permitted to use, so that teams will not be surprized by an enormous eyebolt. 4.c. guarantees that you will have at least 5.0 cm of space between the head of the eyebolt and the bottom of the block. That means that in those 5.0 cm, you need to provide 1/4" clearance for the shaft of the eyebolt. Below that, where the head of the eyebolt and the links of the chain will be, you have to have room for an eyebolt and chain no larger than 3.0 cm.

Looking down at the top of the tower (chimney? smoke stack?) the eyebolt head is not a circle. It is a rectangle with half-round ends. The sides of the triangle aren't solid (I assume), they are representative of cross bracing and will have open spaces between the braces. Suppose that the eyebolt were oriented toward a leg, with the edge of the eyebolt sticking out the side. It will work if the eyebolt doesn't hit a brace. If the chain used is actually 3.0 cm in diameter there could be a conflict with the bracing on the sides, but the links aren't round when seen from above. You should be able to get the chain to hang through the top part of the tower even if there is not a 3.0 cm diameter clear space inside. The chain dimensions in the rules are a worst case and you can get yourself in trouble by pushing the limits too much, but I think there is a misconception that you are required to have a 3.0 cm diameter clear cylindrical volume throughout the tower. The requirement is simply that you have to be able to set up the loading apparatus as described.

Personally, I would not encourage you to build three-legged towers. They are harder to build, they induce bending in the bracing in a direction perpendicular to the faces of the tower, they are usually less efficient than four-legged towers. But, if you can do it, more power to you.

Bob Monetza
Grand Haven, MI

[bmbw123]

For building a three-sided tower, I drew up a design a couple months ago, but ended up deciding to stick with a four-sided on. On the three-sided tower, I ended up calculating the upper section to be an equilateral triangle with sides of about 6.9282 cm (if I did it right haha) with an inner circle with radius of 2 cm, and I'm pretty sure this would match the specifications.

Oh, and one quick question. what kind of cross-sections are you guys using? Currently I'm trying to stick with an x-bracing, but I was hoping to see what everybody else is doing as well.

[Littleboy]

For building a three-sided tower, I drew up a design a couple months ago, but ended up deciding to stick with a four-sided on. On the three-sided tower, I ended up calculating the upper section to be an equilateral triangle with sides of about 6.9282 cm (if I did it right haha) with an inner circle with radius of 2 cm, and I'm pretty sure this would match the specifications.

Oh, and one quick question. what kind of cross-sections are you guys using? Currently I'm trying to stick with an x-bracing, but I was hoping to see what everybody else is doing as well.

X-bracing on base. Z-bracing on chimney. It works very well.

[bmbw123]

X-bracing on base. Z-bracing on chimney. It works very well.

Ok, thanks!

[Littleboy]

Do you guy put glue between where the members of x-bracing intersect? If you do, why? If you do not, why not? Thanks for the help.

[bmbw123]

I do because it seems that it makes it stiffer, and the x-bracings bend around as much. Plus, I don't really see how adding a little bit of glue would really hurt you. I'm quite a novice at this though and haven't done anything like this in years though, so this is just my two cents

[SLM]

Do you guy put glue between where the members of x-bracing intersect? If you do, why? If you do not, why not? Thanks for the help.

From the standpoint of the physics that explains the behavior of such structures, if the bracings are in tension, not much will be gained by gluing them together at their mid point (or any other point). If the bracings are in compression, however, gluing them at their mid-point could strengthen one or both members against buckling.

[bmbw123]

What wood density are you guys making your main side structural supports? I'm looking at around 1.4ish, but with that, I'm not sure how people are able to hold the maximum weight.

[Freyssenet]

What wood density are you guys making your main side structural supports? I'm looking at around 1.4ish, but with that, I'm not sure how people are able to hold the maximum weight.

I am using balsa and I am using specific densities of 0.15-0.17 for the main supports, and 0.12-0.15 for bracing.