General Discussion

[noobforce]

Ok thank you guys! I also have another question. So I tested my tower yesterday and it held about 11kg and broke at the chimney. I'll release the design cause its commonly used anyways /\/\/\/\ and each 1/16 by 1/16 /_\ is spaced about 7 cm. I want to increase the chimney's capability of support to 15 kg. The legs are 3/32 by 3/32 and chimney is 56 cm long and weighs about 5g.

I have thought of a few options and would like to know what is best.

1) Increasing legs to 1/8 by 1/8
2) Another trial because the 3/32 by 3/32 was not perfectly straight
3) Decreasing spacing to about < 6 cm.
4) Adding a horizontal brace midway.

Any other suggestions?

Add a horizontal brace midway. There is no compression brace so the diagonal braces must take compression and tension, but there are not enough diagonal braces for them to manage it all.

I think this would work. I also agree with your idea of increasing the size of the legs. But my best answer would be none of these and say that if you can, I would reccomend you make the tower more proportional so it isn't so top-heavy and it might hold more, at the price of being heavier. Ours has a 40.5 cm top section and a 30 cm base and ends up weighing about 7.5-7.75g usually. Hope this helps!

He is Division C..

[Balsa Man]

Ok thank you guys! I also have another question. So I tested my tower yesterday and it held about 11kg and broke at the chimney. I'll release the design cause its commonly used anyways /\/\/\/\ and each 1/16 by 1/16 /_\ is spaced about 7 cm. I want to increase the chimney's capability of support to 15 kg. The legs are 3/32 by 3/32 and chimney is 56 cm long and weighs about 5g.

I have thought of a few options and would like to know what is best.

1) Increasing legs to 1/8 by 1/8
2) Another trial because the 3/32 by 3/32 was not perfectly straight
3) Decreasing spacing to about < 6 cm.
4) Adding a horizontal brace midway.

Any other suggestions?

Couple quick thoughts/comments:
1) When you say "broke at the chimney", do you know exactly what failed - what the initial failure moode was? If not, you're shooting in the dark. The only reliable way to know that is testing with a safety tower.
2) Remember the basics of column failure - it is 3 factors working together that determine the buckling strength of a column (i.e., your legs); Euler's Buckling Theorem equation; the cross-section (crross-sectional moment of inertia), the modulus of elasticity (which is different from, but is related to density), and the exposed coulom length - the bracing interval (assuming the bracing is working to locate the braced points in 3-D space).

So, going from 3/32nds to 1/8th- if the failure is buckling failure of chimney leg- If at the same density, a) weight will go up by 40+%; strength up by about the same (assuming same bracing interval, and that failure is not a result of bracing failing/not working to stabilize/isolate braced points). So, to gain efficiency (not lose ground from where you are), would want lower density, and/or longer bracing interval, and you'd have to look at the weight tradeoffs
Re-doing at 3/32nds- depends again on what's failing. Not being straight could be the cause- how good/straight is your jigging/building? If things are straight and true, that's not the problem; if they're not, re-doing without fixing that will not help. If things are straight, increasing density will increase strength.
Decreasing bracing interval from 7 to 6cm- the inverse square relationship in Euler's equation says you pick up about 35% in buckling strength with that. That assumes the bracing system you're using is actually working to brace/hold in-place the braced points- so that the leg is functioning as a stacked set of shorter columns. Xs and ladders do accomplish that. The Z-bracing without ladders you describe may not be accomplishing that- back to iinitial, "so exactly how did it fail?" question.
Adding a horizontal brace midway- simply doing one in the middle will not improve anything. Each of the braced sections needs to be providing the required buckling strength.

[flyingwatermelon]

Couple quick thoughts/comments:
1) When you say "broke at the chimney", do you know exactly what failed - what the initial failure moode was? If not, you're shooting in the dark. The only reliable way to know that is testing with a safety tower.
2) Remember the basics of column failure - it is 3 factors working together that determine the buckling strength of a column (i.e., your legs); Euler's Buckling Theorem equation; the cross-section (crross-sectional moment of inertia), the modulus of elasticity (which is different from, but is related to density), and the exposed coulom length - the bracing interval (assuming the bracing is working to locate the braced points in 3-D space).

So, going from 3/32nds to 1/8th- if the failure is buckling failure of chimney leg- If at the same density, a) weight will go up by 40+%; strength up by about the same (assuming same bracing interval, and that failure is not a result of bracing failing/not working to stabilize/isolate braced points). So, to gain efficiency (not lose ground from where you are), would want lower density, and/or longer bracing interval, and you'd have to look at the weight tradeoffs
Re-doing at 3/32nds- depends again on what's failing. Not being straight could be the cause- how good/straight is your jigging/building? If things are straight and true, that's not the problem; if they're not, re-doing without fixing that will not help. If things are straight, increasing density will increase strength.
Decreasing bracing interval from 7 to 6cm- the inverse square relationship in Euler's equation says you pick up about 35% in buckling strength with that. That assumes the bracing system you're using is actually working to brace/hold in-place the braced points- so that the leg is functioning as a stacked set of shorter columns. Xs and ladders do accomplish that. The Z-bracing without ladders you describe may not be accomplishing that- back to iinitial, "so exactly how did it fail?" question.
Adding a horizontal brace midway- simply doing one in the middle will not improve anything. Each of the braced sections needs to be providing the required buckling strength.

So to summarize: 1/8 by 1/8 may improve strength but may reduce efficiency
3/32nd: Probably better to fix bracings and not wood
Decreasing interval: Seems like the more reasonable option to go here
Horizontal bracing midway: Will do nothing. I'll cut that out of my design then.

Thanks! I'll be testing this soon and see how it works. I'm assuming that it broke at the leg, but like you mentioned earlier its hard to tell without setting up the proper testing equipment. I'm not sure if its worth building a safety tower testing rig at this point.

[Balsa Man]

Yeah, a fair summary.
Whether 1/8 would help or hurt depends on density.

Whether decreasing bracing interval, or increasing (3/32nds) leg density will get you to a better score, only testing will tell- both will increase column strength.

You can put a safety tower together in an hour or two. That's a pretty small time investment for what is arguably the most useful/effective "development tool" you can apply to the problem......

[jma]

Our recently built square base has 2 corners of < 90 degree and 2 corners of > 90 degree (about 1 mm) instead of 4 corners of 90 degree, the 28 cm base has 4 x's with 2 horizontal braces ,1 on top & 1 on bottom, we skipped 3 horizontal in the middle. Is there a way to fix that so it'll be perfectly square?

[LKN]

jma, a jig is the best route to get perfect right angles. At this point in time, it cannot really be "fixed" because the tower has already been glued incorrectly. I have had times where I build most of my tower by some type of jig or right angle piece, but once it is removed and additional bracing is applied then the tower can warp slightly due to too high of pre-tension while bracing. For example, the "loop" discussed earlier in the forum describing thin pieces of higher density wood at the bottom of a base to withstand the outward force applied through the tower legs. These pieces can easily manipulate the tower if not glued correctly.

In my past experience the "best", and I mean at best, way to fix a problem once you have made a mistake is to cut the bracings with scissors and take a blade to wedge every so slightly between the bonded pieces of wood. SLOWLY rotate and tweak the bond while holding the leg in place, making sure you don't destroy the tower more than you want to. Once the bracing is off, take fine sandpaper, (in the 200 grit range) and sand away at the glue surface area until the glue has been sanded off. Reapply the bracing with a new piece of wood, keeping everything as constant as you can.

[jma]

Thanks, LKN.
We did use the zig for the base but this time we only used the zig while gluing 1 diagonal of the X on the front of the leg, then we took the base off the zig and glued another diagonal on the back of the leg (we wanted to try this technique that we just learned from Thomsom' s post). We probably warped it when gluing as you mentioned. We never had this problem with other towers so we did not pay attention while gluing this time. We'll try to fix the base as you recomended.

[thsom]

Now I understand that this has absolutely nothing to do with towers, but what is a reasonable efficiency for boomilevers using the formula: grams held/grams of boomilever. I'm talking about for a reasonably competitive regional. My guess is over 1000, but what exactly is definded as poor, fair, good, great, or excellent/fenomenal...

[mrsteven]

Now I understand that this has absolutely nothing to do with towers, but what is a reasonable efficiency for boomilevers using the formula: grams held/grams of boomilever. I'm talking about for a reasonably competitive regional. My guess is over 1000, but what exactly is definded as poor, fair, good, great, or excellent/fenomenal...

lets worry about this next year when we have dimensions or even rumors of dimensions haha its all dependent

[thsom]

Now I understand that this has absolutely nothing to do with towers, but what is a reasonable efficiency for boomilevers using the formula: grams held/grams of boomilever. I'm talking about for a reasonably competitive regional. My guess is over 1000, but what exactly is definded as poor, fair, good, great, or excellent/fenomenal...

lets worry about this next year when we have dimensions or even rumors of dimensions haha its all dependent

lol, this is how i deal with my nostalgia