General Discussion

[Vizard007]

Someone posted rules online, illegally that is. I would just use the rules that your coach will get in order to stay out of any issues/conflicts. Better safe than sorry.

[Faustina]

Someone earlier suggested that "any swing or rotation of the bucket will be brutal." Why would this be, when a swing/rotation of the bucket does not equal a swing/rotation of the loading block?

Also, suppose I have a tower that is 4 cm by 4 cm on top. Does it matter if the loading block is EXACTLY centered, or not?

[dragonfly]

Someone earlier suggested that "any swing or rotation of the bucket will be brutal." Why would this be, when a swing/rotation of the bucket does not equal a swing/rotation of the loading block?

Also, suppose I have a tower that is 4 cm by 4 cm on top. Does it matter if the loading block is EXACTLY centered, or not?

The greater effect that a swing/rotation of the bucket is even more relevant with the added height. As the loading block is connected (of course), it could potentially cause a drastic sway all the way up to the top and lead to easy tipping of the structure. That's a simple explanation, at least. Either way, a smooth and level loading process is absolutely always preferred, and a tower that is either perfectly constructed or prepared with design elements against tipping.

The loading block should be distributing the same amount of force anywhere on its surface....that isn't your problem. However, I still would suggest ensuring good centering for testing, as you never know what the chain might do during testing or anything else. Plus, it's good practice, and ensures that all legs are comfortably under the block.

[rjm]

I made the comment about the swinging/rotating bucket. If the bucket is allowed to swing like a pendulum, then the lateral movement of the mass is resisted by the friction between the loading block and the tops of the tower legs, and those in turn are held from lateral movement by stresses distributed through the structure. The taller the structure, the greater the leverage between the top of the tower and the base. Similarly, if the bucket is allowed to rotate and clunk against the test apparatus, a torsional resistance is required throughout the tower and again, a taller tower will have more tendency to twist.

If you can load very slowly, these movements may never be great enough to matter. If not, then there should be some way to dampen the movement.

Centering the block between the tops of the legs is preferred. If the eyebolt is closer to one leg than the others, it will impose more load on that leg. (It's the same principle as statically loading a simple beam with an off-center load, in which the reaction is R=aP/l where a=distance from P to the opposite support, l=span between supports, and P=load. For a centered load, R=P/2.) The effect is probably pretty small in this case.

Bob Monetza
Grand Haven, MI

[iwonder]

I know this is off-topic, and kind of dead... however, I would love to see a smartphone app, and seeing that you currently pay $8(minus shipping) for 23 sets of rules, I would pay 50 cents per set of rules, if the app was free, cheap I know, but we're mostly middle-highschool students.

Now, on topic...

Has anyone seen a tendency in their towers to begin to rotate(around the vertical axis) when a load is applied? I understand why my design does this(I think), but I've never even heard of this before, it seems strange.

[SLM]

Has anyone seen a tendency in their towers to begin to rotate(around the vertical axis) when a load is applied? I understand why my design does this(I think), but I've never even heard of this before, it seems strange.

I think you are referring to torsion, something like the behavior shown in the following image?


This kind of behavior is not uncommon in tall structures. It is generally due to unsymmetrical geometry and/or loading. We experienced this kind of behavior with our three-legged tower last year.

If your tower has four legs and its chimney (core) is a rectangular prism, then you may want to closely examine your construction technique, especially where the core and the base meet. If the load from the core is not distributed uniformly among the four legs, then your tower may end up twisting.

[iwonder]

This is the first 3 legged tower our school has built, so we don't hav much experience with them. Thanks for the help,

[SLM]

Someone earlier suggested that "any swing or rotation of the bucket will be brutal." Why would this be, when a swing/rotation of the bucket does not equal a swing/rotation of the loading block?

Here is a simple computer simulation that shows the effect of the bucket swing on the tower. More specifically, the simulation involves two identical towers, except for their heights, each supporting a suspended load bucket. Note how the taller tower tends to have a larger lateral movement, compared to the shorter tower, for the same swing of the bucket.

Click on the image below to see the simulation.

[chalker]

Here is a simple computer simulation that shows the effect of the bucket swing on the tower. More specifically, the simulation involves two identical towers, except for their heights, each supporting a suspended load bucket. Note how the taller tower tends to have a larger lateral movement, compared to the shorter tower, for the same swing of the bucket.

Very cool simulation. Can you tell me what program you used to create it?

Also, everyone should keep in mind that all of this discussion about 'swinging' is greatly oversimplifying what is actually going on. As the animation shows, it assumes the chain connecting the bucket to the block is a rigid piece that can only flex at the point of contact with the loading block. In reality there are lots of points it can flex. Minor swinging is likely to be contained mostly to the bucket itself.

[SLM]

Very cool simulation. Can you tell me what program you used to create it?

Physion (physion.net).