Scioly.org

JojoCho wrote:

Complexity wrote:

Complexity wrote:
Every 1/12 of the tower. At every six inches. The buckling strength is 20 for a piece. It is 15 for the rest, so less. My legs are very light, so I will take the floppy leg more bracing approach.

If you say that 1/7 will work, then I think I will reconsider my bracing interval. I want a little more to be safe, maybe 1/10. 1/7 worked with 20 buckling strength, but would more would probably be needed since now they all show 15.
Made the cutting jig

1/7 with be ok, you could actually probably even use 1/16 balsa sticks, that would reduce your tower weight as well.

All right, thanks for the help. I am cutting down on bracings. The reason I chose to pursue 3/32 legs was because I have an invite tomorrow and could not find 1/8 in any store. Glad to hear that I still have a chance.

Complexity wrote:

Complexity wrote:

Balsa Man wrote:Actually, with that buckling strength, bracing at 1/7 intervals with ladders and Xs should work. What interval have you been using?

Every 1/12 of the tower. At every six inches. The buckling strength is 20 for a piece. It is 15 for the rest, so less. My legs are very light, so I will take the floppy leg more bracing approach.

If you say that 1/7 will work, then I think I will reconsider my bracing interval. I want a little more to be safe, maybe 1/10. 1/7 worked with 20 buckling strength, but would more would probably be needed since now they all show 15.
Made the cutting jig

Complexity, there are a couple things I’m having trouble following, and/or that aren’t making sense; let’s see if we can figure them out-.
“1/7 worked with 20 buckling strength”
What interval are you using? “1/12th of the tower”
“The buckling strength is 20 for a piece. It is 15 for the rest, so less”

You say you were bracing at 1/12 interval, and say that’s every 6 inches. For a C tower, leg length will be about 61.5cm for a tower meeting the 29cm circle bonus, for one just clearing the 20cm x 20cm base opening, leg length will be around 61cm. 1/12 (your bracing interval) of 61cm is 5.08cm; braced points every 5.08cm along the legs. So, even if you meant 6cm, instead of inches, bracing interval number isn’t correct. The braced interval for 1/7, on a 61cm leg is 61/7 = 8.7cm.

Let’s review some calculations. 1/12 interval first.
For (36”) leg sticks with scale reading from single finger push-down testing ; scale reading of 20gr. That’s buckling strength under pinned/pinned end conditions. So, multiply that by 2.3 (to calculate buckling strength when in fixed/fixed end conditions – see discussion in the thread ‘measuring/using buckling strength-new info’) , you get 46gr. So, how does bracing a stick 36” long that buckles at 20gr to shorter intervals (5.08cm) increase that? 5.08cm is 0.0555 of 91.6cm (36”). Inverse square relationship tells us: 1/0.0555 squared is 325.135. 46gr x 325.135 is 14,926gr. That’s of course, WAY over what’s needed (3810, plus a safety factor; at a 20% SF, that takes us to 4572gr). But, still way over; >3x what you need. Change the 36” buckling strength measured on the scale to 15 (instead of 20). Times 2.3, that becomes 34.5gr. 34.5 x 325.135 = 11,217gr. Again, way over; >twice what you need.

Ok, lets run the same numbers for 1/7 interval bracing. 1/7 of 61 is 8.7cm 8.7 is 0.095 of 91.6. 1/0.095 squared is 110.8. For a 36” stick testing on the scale at 20gr, 20 x 2.3 = 46, and 46 x 110.8 = 5,097gr substantially over 3,810, and 4,572.
If the 36” buckling scale reading is 15, 2.3 x 15 = 34.5gr. 34.5gr x 110.8 = 3,822gr, just a hair below design load of 3,810gr, no safety factor = will likely not get you to a 15kg tower load. So, with 3/32nd”, 36” sticks that only test at 15, you’ll need to take bracing interval down to 1/8.

1/8 of 61cm = 7.625cm. 7.635cm is 0.0832 of 91.6cm. 1/0.0832 squared = 144.315. 34.5gr x 144.315 = 4,979gr; enough, with a safety factor of a bit over 20%.

The numbers say using 36” sticks testing on the scale at 15gr for legs, braced at 1/8 intervals should work fine.

But, one other important thing to check. Calculations above are for legs. One other design factor that has to work, too; the buckling strength of the ladders (especially the lower (longer) ladders.
I think from what you said, you used 3/32nd” stick, scale testing at 15g. As I’ve discussed, in a ladders and Xs setup, the design load the ladders need to carry is only about 1kg. I’ve explained this is …an experience-based number (while the leg design load is an actually calculated number). Calculating ladder buckling strength (for 36” stick testing at 15gr) is done the same was as for legs. Without taking the time to draw it up, not sure what your lowest ladder length is; I’m guessing a bit under 19cm, I’ll use 18.75. 18.75 is 0.205 of 91.6cm ; 1/0.205 squared is 23.87. 23.87 x 34.5gr = 824gr. Because the 1kg ‘design load’ is just a working estimate, this might work, but I suspect it won’t. At ladder lengths at/below about 16.5cm, it will. For a ladder at 18.75, you’ll need a 36” test value of 22-23gr.

I’m curious; did you build/test tower at the 20 gr at 36” braced at 1/12 interval; what did it weigh, what did it carry? Same question on the one braced at 1/7 that ‘worked.’

Hope this helps, and helps understand the inverse square calculation. Good luck!

JojoCho wrote:

Complexity wrote:

Complexity wrote:
Every 1/12 of the tower. At every six inches. The buckling strength is 20 for a piece. It is 15 for the rest, so less. My legs are very light, so I will take the floppy leg more bracing approach.

If you say that 1/7 will work, then I think I will reconsider my bracing interval. I want a little more to be safe, maybe 1/10. 1/7 worked with 20 buckling strength, but would more would probably be needed since now they all show 15.
Made the cutting jig

1/7 with be ok, you could actually probably even use 1/16 balsa sticks, that would reduce your tower weight as well.

See post above for detailed analysis. As discussed, with 36" sticks testing on the scale at only 15gr, you are going to have to go to a 1/8 interval. 1/16" sticks for legs, to have enough buckling strength for use in a braced at 1/7 (or 1/8) intervals setup would have to be of very high density- I don't think even at 1.1, 1.2gr/36 (which is the highest density Specialized balsa carries, and balsa exist in (and we're talking $5/$7 per stick)) you'd see a 36" buckling reading of 15gr.

Balsa Man wrote:

Complexity wrote:

Complexity wrote:
Every 1/12 of the tower. At every six inches. The buckling strength is 20 for a piece. It is 15 for the rest, so less. My legs are very light, so I will take the floppy leg more bracing approach.

If you say that 1/7 will work, then I think I will reconsider my bracing interval. I want a little more to be safe, maybe 1/10. 1/7 worked with 20 buckling strength, but would more would probably be needed since now they all show 15.
Made the cutting jig

Complexity, there are a couple things I’m having trouble following, and/or that aren’t making sense; let’s see if we can figure them out-.
“1/7 worked with 20 buckling strength”
What interval are you using? “1/12th of the tower”
“The buckling strength is 20 for a piece. It is 15 for the rest, so less”

You say you were bracing at 1/12 interval, and say that’s every 6 inches. For a C tower, leg length will be about 61.5cm for a tower meeting the 29cm circle bonus, for one just clearing the 20cm x 20cm base opening, leg length will be around 61cm. 1/12 (your bracing interval) of 61cm is 5.08cm; braced points every 5.08cm along the legs. So, even if you meant 6cm, instead of inches, bracing interval number isn’t correct. The braced interval for 1/7, on a 61cm leg is 61/7 = 8.7cm.

Let’s review some calculations. 1/12 interval first.
For (36”) leg sticks with scale reading from single finger push-down testing ; scale reading of 20gr. That’s buckling strength under pinned/pinned end conditions. So, multiply that by 2.3 (to calculate buckling strength when in fixed/fixed end conditions – see discussion in the thread ‘measuring/using buckling strength-new info’) , you get 46gr. So, how does bracing a stick 36” long that buckles at 20gr to shorter intervals (5.08cm) increase that? 5.08cm is 0.0555 of 91.6cm (36”). Inverse square relationship tells us: 1/0.0555 squared is 325.135. 46gr x 325.135 is 14,926gr. That’s of course, WAY over what’s needed (3810, plus a safety factor; at a 20% SF, that takes us to 4572gr). But, still way over; >3x what you need. Change the 36” buckling strength measured on the scale to 15 (instead of 20). Times 2.3, that becomes 34.5gr. 34.5 x 325.135 = 11,217gr. Again, way over; >twice what you need.

Ok, lets run the same numbers for 1/7 interval bracing. 1/7 of 61 is 8.7cm 8.7 is 0.095 of 91.6. 1/0.095 squared is 110.8. For a 36” stick testing on the scale at 20gr, 20 x 2.3 = 46, and 46 x 110.8 = 5,097gr substantially over 3,810, and 4,572.
If the 36” buckling scale reading is 15, 2.3 x 15 = 34.5gr. 34.5gr x 110.8 = 3,822gr, just a hair below design load of 3,810gr, no safety factor = will likely not get you to a 15kg tower load. So, with 3/32nd”, 36” sticks that only test at 15, you’ll need to take bracing interval down to 1/8.

1/8 of 61cm = 7.625cm. 7.635cm is 0.0832 of 91.6cm. 1/0.0832 squared = 144.315. 34.5gr x 144.315 = 4,979gr; enough, with a safety factor of a bit over 20%.

The numbers say using 36” sticks testing on the scale at 15gr for legs, braced at 1/8 intervals should work fine.

But, one other important thing to check. Calculations above are for legs. One other design factor that has to work, too; the buckling strength of the ladders (especially the lower (longer) ladders.
I think from what you said, you used 3/32nd” stick, scale testing at 15g. As I’ve discussed, in a ladders and Xs setup, the design load the ladders need to carry is only about 1kg. I’ve explained this is …an experience-based number (while the leg design load is an actually calculated number). Calculating ladder buckling strength (for 36” stick testing at 15gr) is done the same was as for legs. Without taking the time to draw it up, not sure what your lowest ladder length is; I’m guessing a bit under 19cm, I’ll use 18.75. 18.75 is 0.205 of 91.6cm ; 1/0.205 squared is 23.87. 23.87 x 34.5gr = 824gr. Because the 1kg ‘design load’ is just a working estimate, this might work, but I suspect it won’t. At ladder lengths at/below about 16.5cm, it will. For a ladder at 18.75, you’ll need a 36” test value of 22-23gr.

I’m curious; did you build/test tower at the 20 gr at 36” braced at 1/12 interval; what did it weigh, what did it carry? Same question on the one braced at 1/7 that ‘worked.’

Hope this helps, and helps understand the inverse square calculation. Good luck!

First off, Balsa Man, I think I owe you an apology. I must have been really confusing with my typos and wording. (Got to start proofreading these posts). What I meant was to originally build a tower with 3/32 legs and brace it at every 1/12 of the tower (about 5cm like you said. Dont know why I said 6in.). However, it was realized that this was far too much, and scrapped that idea. You said that 1/7 could work, and then I responded by saying that I was incorrect about the buckling strength of my leg pieces- only one of them had a buckling strength of 20, the other three had a buckling strength of 15.. This changed the numbers, and now I know to brace my tower at 1/8. This tower we are talking about has not been built yet, I am in the construction phase right now. I wanted to calculate my bracing interval on these forums before randomely heading into the construction. Currently I am building the tower, and it will have a bracing interval of 1/8- as your calculations have proved. I will make sure to measure my bracings and completed tower weight. I am testing it tomorrow. What I meant by the one braced at 1/7 that worked was that it exceeded 3970 in the calculations-capable of supporting load- I did not actually build and test it.

However, I have two questions. Is there use of ladders at the bottom of the tower (Like within a cm of the table surface but not touching the ground). And, if the ladder bracing has to have a BS of 22-23 grams, how can we brace the legs with 1/16 pieces. I cannot reach that BS with even my 3/32 legs. What I mean is that I dont have a piece for bracings that exceeds 23 grams. I am cutting 1/32 by 1/16 pieces for X and I was planning on doing ladders with 1/16 by 1/16. None of them have a BS of 23.

Hey, glad we got things figured out- understand typos/etc.
Understand your situation, and what you've done.

You're on the right track.

As to your followup questions-
At the bottom of the tower- there is some force pushing lower leg ends outward. Your lowest X set should have the low ends of the X strips attached to lower leg ends near the base- like no more than ~1/8" up. That MAY provide the needed bracing to hold them/keep them from kicking out. I'm actually awaiting a test by one of the teams I'm working with. What they've done is, in addition to lower Xs, put a 'band' around the base- all 4 legs- same specs as the X strips (1/64th" x 1/16")- put on about 1/16' above the base plane. Plan is to load to close to 15kg- legs are ....extra-strong (~25% above design load), and clip the band strips when its under load, to see if the Xs alone can hold things together.

Using 1/16, as I said in my reply to what JoJoCho posted, IMHO, no way, legs or ladders.

On ladder buckling strength, as I said, it's only ladder(s) over ~16.5cm where 3/32" testing at 15gr won't work. With competition coming up (and no time to get 3/32 stiff enough), So, for your bottom ladder, and 2nd if its >16.5cm) I'd take a second piece of 3/32, about 1/2, might get away w/ 1/3 of the ladder length, and laminate it, centered, to the low/lonng ladder(s)- yeah, a bit of a weight penalty, but should give the needed strength

Balsa Man wrote:Hey, glad we got things figured out- understand typos/etc.
Understand your situation, and what you've done.

You're on the right track.

As to your followup questions-
At the bottom of the tower- there is some force pushing lower leg ends outward. Your lowest X set should have the low ends of the X strips attached to lower leg ends near the base- like no more than ~1/8" up. That MAY provide the needed bracing to hold them/keep them from kicking out. I'm actually awaiting a test by one of the teams I'm working with. What they've done is, in addition to lower Xs, put a 'band' around the base- all 4 legs- same specs as the X strips (1/64th" x 1/16")- put on about 1/16' above the base plane. Plan is to load to close to 15kg- legs are ....extra-strong (~25% above design load), and clip the band strips when its under load, to see if the Xs alone can hold things together.

Using 1/16, as I said in my reply to what JoJoCho posted, IMHO, no way, legs or ladders.

On ladder buckling strength, as I said, it's only ladder(s) over ~16.5cm where 3/32" testing at 15gr won't work. With competition coming up (and no time to get 3/32 stiff enough), So, for your bottom ladder, and 2nd if its >16.5cm) I'd take a second piece of 3/32, about 1/2, might get away w/ 1/3 of the ladder length, and laminate it, centered, to the low/lonng ladder(s)- yeah, a bit of a weight penalty, but should give the needed strength

All right, thanks a lot Balsa Man. You answered all my questions, and all that is left for me is to now construct the tower. I will laminate my lower ladder with the information you gave me. Glad I came here, or else I would be carrying a tower with tons of bracings. It really caught my attention when you talked about ladders and x'es. I have been using Z's almost all season, and have achieved decent efficiencies. I needed a little more information, so I made an account last week

You're very welcome; its a pleasure to be able to be of some help.

Scioly forums are a very cool resource

You're on the right track, good luck; let us know how it goes.

Can you reuse towers that have already been tested and held it all?

candle 1586 wrote:Can you reuse towers that have already been tested and held it all?

You certainly can, as in you're allowed to. The question is, do you really want to? Some things to think about in making that decision:
Prepping for your other S.O. events, time demands from school and other things could make it seem like a good idea.
Whether it will hold full load a second time depends, really, on how close to the limit you're pushing things with your design, and construction. You may not have, and/or may not be able to get a decent read on that. If there's no visible damage (looking really closely at every piece and joint), it may be fine, but there could be damage you can't see; joint(s) where the glue has partially cracked/let go; internal wood fibres broken/separated. The more competitive, generally the lower the chance of it carrying full load again. If it is a reasonably well built 20gram tower, and you see no damage, its probably good for another go. If its a 9 or 10gram tower, significantly less likely.If it sat there like a rock as you loaded on 15KG, more likely; if it was 'talking' (making noises), or you could see any distortions coming in as you got toward full load, less likely. If you're happy with score, and where it might place next competition, just be aware that many of your competitors will have upped their game. With it holding full load, you know you can do better (if you understand where it is 'over engineered').

If you're up for trying to improve it, and have the time, you want to try to figure out how over-engineered it is- load it till it breaks, and do that testing with a way to know what lets go first; high frame rate/slow motion video can do that; another way is using some form of a "safety tower" - connecting the top end of the eyebolt (with a connecting nut) to something - a point above the top of the tower, an inverted eyebolt hung from something that's strong enough to hold (?) 20kg.The first thing to break is your weakest point/member. Let's say a leg fails between bracing points. Easiest way to bring down the weight is reducing the density. If, say, it goes to 18kg; that's ~20% over. Density to buckling strength relationship is roughly linear. So, say for example, your legs are from 36" 1/8 x 1/8" sticks weighing...1.8gr. 80% of that's 1.44gr, so you could use 1.44, 1.45, 1.5gr/36" sticks.

Hi everyone,

First time builder here (actually just assisting my 9th grade son). I've read through all the tips and information. Very helpful, although limited in practical use when I can't even get the kid to measure carefully or cut in a straight line.

Anyway, one question: when building ladders and X's, is it necessary or helpful to have a ladder rung at the base of the tower legs? In my mind it seems like it could be omitted, as the bottom of the legs are resting on the table, and would be more likely to try to slide out as the tower is loaded than anything else. Is this correct? There would be X's in the bottom section.

Thanks for your assistance.