Towers B/C

[Balsa Man]

So I built a pretty good tower, 9.1kg and bonus with 7.7g but the failure point surprised me. It broke at the very top ladder on both of the towers I tested today (the other one held 9.1kg with 9.62g, so I know what the failure point is pretty accurately). I'm wondering what to do to fix this. Should I use heavier wood? Should these ladders be in direct contact with the block or not? Any other tips tor reduce the high compressive forces here?

Since the ladder is intended to take primarily horizontal compression, what if you placed it on top of the legs instead? This would (maybe?) allow the load to act as a compression force on the legs, rather than shearing the ladder.

As discussed before, ladders work best butt jointed between the legs (so that if/as legs try/start to buckle inward, that force is put into the ladder axially). Its pretty much critical the top ladder be done this way, NOT put on top of the leg ends. At the top, the ladders are not resisting buckling, they’re holding the leg tops apart; there is a force pushing the leg tops together, because the legs are angled from vertical. Get’s up to something over 1kg by full load. If the ladder is sitting on top of the leg ends, then the leg ends are butt jointed to the underside of the ladder- a very weak joint- force of two adjacent leg tops together could very easily cause that joint to fail- thin layer of underside of the ladder shears off, down things go…. Also, ladder sitting on top is subject to crushing, unless its….much higher density than it needs to be. All in all, a bad idea; sorry. Ladder should be between the legs, with, oh, 1-2mm of leg sticking up above the top of ladder (to allow final sanding of the top to get all 4 legs in the same plane, and load block parallel to test base).

What I can’t tell from Tesel’s failure description is how it broke “at the very top ladder.” Something….pretty fundamentally has to be wrong, it sounds like….

From previous discussions, I assume/understand a) jig was modified so that the leg tops are positioned close enough they are fully underneath the load block (i.e., distance between the outsides of two adjacent legs is <5.0cm), b) ladders were properly butt jointed between the legs, and c) ladders were from 1/8” wood. If this is correct, it should have worked fine, not failed at 60% load. Was it a ladder actually failing under buckling load (i.e., broken in the middle), or one end popping loose (joint failure), or leg breaking just under the ladder, or… something else? Was the/a ladder damaged (dented/nicked) before testing? Was the failure simply observed in real time (vs from video)? Any additional clues/info could help get to the bottom of the mystery.

[Balsa Man]

So I was watching Towers at a recent Regional. Unless I saw this wrong, a supervisor had his test base sitting there, got out a circular piece of paper (dia ~29 cm) with a ~20 x 20 cm hole in the middle, laid it on top of the test base, and traced the circle onto the test base. I thought this was odd because if the circle was exactly 29 cm, the resulting traced outline by definition had to be more than that. If he had calculated the exact width his pen mark would make and made the template small enough to result in a 29 cm circle when traced, well, then, good for him, but my kids' tower didn't fit the bonus circle even though it did the night before on our base (which I confirmed via ruler today). Again, I could have misinterpreted what he was doing, but I have two questions:

1. Is there anyway that circle could have been accurate?
2. Are the kids allowed to ask the circle to be measured?

This is actually exactly what I did to draw the bonus circle on my school's test base at the beginning of the year, although my paper was printed out in four pieces and taped together, and I made sure to account for pencil line width. You can probably ask that the circle be measured, but probably nothing will come of it even if it's discovered to be inaccurate (at least that's what would happen where I am, not sure how your tournament will respond).

Its certainly possible the circle ended up at the correct diameter, but it sounds like it wasn't (given confirmation measuring). Possible difference in rulers, but not very likely. If the circular piece of paper was accurate (i.e., 29.0cm), then the inside of the line traced onto the base would be quite accurate, and the check to see if meeting the circle bonus would/should have been anything extending past the inside edge of the line.

Absolutely, kid or coach can, and should have, checked. Best to do this first thing in the morning (to take away the... pretty compelling argument, "well, you're right its wrong, its 29.2cm, but, hey, 10 kids have already run, and classified as meeting or not meeting the bonus with the incorrect circle, so changing for you at this point just wouldn't be fair..."). Its a philosophical thing with me, but I believe strongly it is really important when there is a....dimensional rule, where meeting or not meeting it has scoring/placing ramifications, that the folk running the event do what they have to do to be correct/accurate. The kids have to follow the rules- so do event supervisors. With volunteer labor, its understandable you might end up with a supervisor that ...for whatever reason doesn't meet that standard. If a kid/coach can show/shows a dimensional check is wrong - the fairness standard cutting the other way - the event folk should acknowledge and adjust fairly- e.g., "oh, yeah, its the inside of our line that's at 29.0, not the outside", or....whatever. The idea that if event supervisor knows something they did/are doing is wrong, before anyone has competed/been classified, and they....just blow it off, is just plain wrong in my book, and should not happen.

[jgrischow1]

So I was watching Towers at a recent Regional. Unless I saw this wrong, a supervisor had his test base sitting there, got out a circular piece of paper (dia ~29 cm) with a ~20 x 20 cm hole in the middle, laid it on top of the test base, and traced the circle onto the test base. I thought this was odd because if the circle was exactly 29 cm, the resulting traced outline by definition had to be more than that. If he had calculated the exact width his pen mark would make and made the template small enough to result in a 29 cm circle when traced, well, then, good for him, but my kids' tower didn't fit the bonus circle even though it did the night before on our base (which I confirmed via ruler today). Again, I could have misinterpreted what he was doing, but I have two questions:

1. Is there anyway that circle could have been accurate?
2. Are the kids allowed to ask the circle to be measured?

This is actually exactly what I did to draw the bonus circle on my school's test base at the beginning of the year, although my paper was printed out in four pieces and taped together, and I made sure to account for pencil line width. You can probably ask that the circle be measured, but probably nothing will come of it even if it's discovered to be inaccurate (at least that's what would happen where I am, not sure how your tournament will respond).

Its certainly possible the circle ended up at the correct diameter, but it sounds like it wasn't (given confirmation measuring). Possible difference in rulers, but not very likely. If the circular piece of paper was accurate (i.e., 29.0cm), then the inside of the line traced onto the base would be quite accurate, and the check to see if meeting the circle bonus would/should have been anything extending past the inside edge of the line.

Absolutely, kid or coach can, and should have, checked. Best to do this first thing in the morning (to take away the... pretty compelling argument, "well, you're right its wrong, its 29.2cm, but, hey, 10 kids have already run, and classified as meeting or not meeting the bonus with the incorrect circle, so changing for you at this point just wouldn't be fair..."). Its a philosophical thing with me, but I believe strongly it is really important when there is a....dimensional rule, where meeting or not meeting it has scoring/placing ramifications, that the folk running the event do what they have to do to be correct/accurate. The kids have to follow the rules- so do event supervisors. With volunteer labor, its understandable you might end up with a supervisor that ...for whatever reason doesn't meet that standard. If a kid/coach can show/shows a dimensional check is wrong - the fairness standard cutting the other way - the event folk should acknowledge and adjust fairly- e.g., "oh, yeah, its the inside of our line that's at 29.0, not the outside", or....whatever. The idea that if event supervisor knows something they did/are doing is wrong, before anyone has competed/been classified, and they....just blow it off, is just plain wrong in my book, and should not happen.

Yeah, I guess I didn't even see if they were using the circle they traced as the "OK zone" and the inside of THAT as the no-go zone. As a coach since they already entered the competition area I didn't think it was OK to ask/check but I was silently hoping my kids would have asked for a confirmation.

[Tesel]

So I built a pretty good tower, 9.1kg and bonus with 7.7g but the failure point surprised me. It broke at the very top ladder on both of the towers I tested today (the other one held 9.1kg with 9.62g, so I know what the failure point is pretty accurately). I'm wondering what to do to fix this. Should I use heavier wood? Should these ladders be in direct contact with the block or not? Any other tips tor reduce the high compressive forces here?

Since the ladder is intended to take primarily horizontal compression, what if you placed it on top of the legs instead? This would (maybe?) allow the load to act as a compression force on the legs, rather than shearing the ladder.

As discussed before, ladders work best butt jointed between the legs (so that if/as legs try/start to buckle inward, that force is put into the ladder axially). Its pretty much critical the top ladder be done this way, NOT put on top of the leg ends. At the top, the ladders are not resisting buckling, they’re holding the leg tops apart; there is a force pushing the leg tops together, because the legs are angled from vertical. Get’s up to something over 1kg by full load. If the ladder is sitting on top of the leg ends, then the leg ends are butt jointed to the underside of the ladder- a very weak joint- force of two adjacent leg tops together could very easily cause that joint to fail- thin layer of underside of the ladder shears off, down things go…. Also, ladder sitting on top is subject to crushing, unless its….much higher density than it needs to be. All in all, a bad idea; sorry. Ladder should be between the legs, with, oh, 1-2mm of leg sticking up above the top of ladder (to allow final sanding of the top to get all 4 legs in the same plane, and load block parallel to test base).

What I can’t tell from Tesel’s failure description is how it broke “at the very top ladder.” Something….pretty fundamentally has to be wrong, it sounds like….

From previous discussions, I assume/understand a) jig was modified so that the leg tops are positioned close enough they are fully underneath the load block (i.e., distance between the outsides of two adjacent legs is <5.0cm), b) ladders were properly butt jointed between the legs, and c) ladders were from 1/8” wood. If this is correct, it should have worked fine, not failed at 60% load. Was it a ladder actually failing under buckling load (i.e., broken in the middle), or one end popping loose (joint failure), or leg breaking just under the ladder, or… something else? Was the/a ladder damaged (dented/nicked) before testing? Was the failure simply observed in real time (vs from video)? Any additional clues/info could help get to the bottom of the mystery.

You're correct. On both of the towers, the ladders were butt jointed. All 4 legs were directly touching the block, but the ladders were not actually in contact. Should they be in contact, or should I leave some space to sand down legs and just have the legs in contact? Unfortunately, the only other person there was bracing the bucket, so there's no video recording. From what it sounds like, I need to try to make another tower to test and video record before I make my final one for Saturday.

[Balsa Man]

As I said, ladders should be just a hair below upper leg ends (" Ladder should be between the legs, with, oh, 1-2mm of leg sticking up above the top of ladder (to allow final sanding of the top to get all 4 legs in the same plane, and load block parallel to test base").. So, no problem there. Assume the tops were sanded to full, even 4 point, level contact. If so, neither ladder installation nor how the block's riding the leg tops sound like any problem. Just have to watch carefully next test.

[Gol17]

Is 1/8 x 1/8 the smallest size that can be used for legs for a div B tower ? I made a tower with 2.6 gms 36 inch sticks for legs and it's too heavy. If I sand down the legs to a 3/32 x 3/32 cross section, will that hurt the buckling strength of the legs ?

[Balsa Man]

Is 1/8 x 1/8 the smallest size that can be used for legs for a div B tower ? I made a tower with 2.6 gms 36 inch sticks for legs and it's too heavy. If I sand down the legs to a 3/32 x 3/32 cross section, will that hurt the buckling strength of the legs ?

If you dig back into the pages of posts, you'll find the answers to how buckling strength works, and your questions. Briefly/simply:
No, 1/8 x 1/8 is not the smallest. You could conceivably use 1/16 x 1/16, but it would have to be of pretty high density and/or braced at a very tight interval (to up the buckling strength sufficiently).
Yes, sanding down will reduce buckling strength. BS is a function of cross section ("I" in Euler's equation) times inherent stiffness (modulus of elasticity, "E" in Euler's equation), divided by length squared. With cross section down 3/4 of the cross section at 1/8", BS would be about 3/4 of what it was at 1/8 cross section.

[Gol17]

Thanks Balsa Man.

I have been reading the posts and gathered that 1.5 gr / 36" stick would be good enough to hold 15 kgs (with X braces). Since the wood I have is high density 3/4 strength of 1/8" would work for me I guess. Understand that BS will depend on how I brace the columns as well. Now if only I can find the ideal interval for bracing..

[fdf4]

Should the loading block be in contact with the very first ladder? Or should it only contact the four bits that extend off each leg past the first ladder?

[fdf4]

Should the loading block be in contact with the very first ladder? Or should it only contact the four bits that extend off each leg past the first ladder?

Nevermind, this was already answered! Can anyone explain why though?