Towers B/C

[fdf4]

What exactly are ladders in regards to a tower or bracing?

[Balsa Man]

What exactly are ladders in regards to a tower or bracing?

The horizontal bracing pieces between legs. Diagonal braces are referred to as X braces (two crossed) or Z braces (single diagonal)

You might want to take the time to go back through posts and.....come up to speed. All the questions you're asking (and lots of questions you haven't asked yet) are answered in lots of detail. A lot of knowledge just waiting for you to explore it....

[fdf4]

Maybe. I just wasn't keen on reading through 500+ posts

[Balsa Man]

Understand, its a bit intimidating.
But if there are things you're trying/wanting to understand to do better, research is the only way....
There's nowhere else that has anything like the compendium of detailed info, focused to 2017 SO towers. That's just....how it is. If you do take the time, it will pay off for you.

[dhdarren]

Hey, glad we got things figured out- understand typos/etc.

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.

Hi Balsa Man,

Do you have any update on how well this method for the bottom of the tower works? I built my tower last night with on 4 tension strips on the bottom instead of ladders, and due to time constraints, I won't have time to actually test it and build another before the upcoming competition. But, after building, I'm having a little bit of doubt about if it will hold up, and am considering adding in ladders on the bottom. Since ladders are anchored flat against the leg, don't they prevent any movement of the legs that might result in buckling? I'm worried about buckling that might happen first on the lowest segment of the tower.

[Random Human]

I think he means a tower with 4 legs vs a tower with 3 legs

Pyramid tower- a tower that looks a bit like this -> http://kidder.ca/media/catalog/product/ ... 48655x.jpg
what he means is that all four legs lean towards the center

3 sided towers could generally be "a pyramid"

When replying to this post, a questions popped up...

Would there be any issue with creating a 3 legged tower? -that's legs are using equilateral triangle (60 degrees) of sticks, how a 1/8*1/8 is a square, the legs being in a shape of an equilateral triangle?

Is there any issues with a design as such

This question has already been asked/answered a couple of times this year. The main disadvantage comes in the bracing. In a square tower, the faces of adjacent legs are parallel to each other. A ladder, put between two legs sees axial loading when/if one of the legs starts/tries to buckle in toward the adjacent leg- the compressive force is straight along its long axis. With the leg faces at 30 degree angle, that same beginning of buckling puts a non-axial load on; the force is at an angle, and it acts immediately to start bowing the ladder; starting the ladder buckling. So its buckling strength is MUCH lower than if it were under axial loading; need a much stiffer (i.e., much heavier) ladder to hold against the force. The other issue is getting square load block properly centered on the top leg end triangle- so that the center, the eyebolt, is centered between the legs. If its off at all, one leg gets disproportional loading, breaks early.

Balsa Man- perhaps you mid understood my question.
Understood that the bracing would not have much points of contact with the leg itself
my idea is to use 60 degree equilateral triangle leg sticks, described in my post above?
any thoughts?

[Alke]

Hey,

This is my first year doing Science Olympiad and also on the forums. I have no engineering experience whatsoever. I am in Division C in Virginia and at regionals, I received a first place in Eletric Vehicle so my coach put me on the States Team. As a result, I was just assigned Towers. My school has not been doing well in Towers - 7th at regionals. I read through the whole 2011, 2012, and 2017 forums and came to the conclusion to use 1/8th square balsa for the legs and 1/5th bracing. For bracing, I plan to use 1/16 square for the Xs and ladders. I am trying to get the bonus. I have not built anything. Should I start building a 2D or 3D jig? What the best way to start building a 1st Place States Tower with my time contraints (3 weeks and 2 weekends)? Thank you!!

[Alke]

In retrospect, I sound cocky but I would like to clarify. I am trying to win States and would appreciate all of your guys help. This is my first year, so my expectations maybe overly optimistic. To clarify on my tower. The base is 21 centimeters square and similar to the one Tessel built. Thank you.

[Balsa Man]

Pyramid tower- a tower that looks a bit like this -> http://kidder.ca/media/catalog/product/ ... 48655x.jpg
what he means is that all four legs lean towards the center

3 sided towers could generally be "a pyramid"

When replying to this post, a questions popped up...

Would there be any issue with creating a 3 legged tower? -that's legs are using equilateral triangle (60 degrees) of sticks, how a 1/8*1/8 is a square, the legs being in a shape of an equilateral triangle?

Is there any issues with a design as such

This question has already been asked/answered a couple of times this year. The main disadvantage comes in the bracing. In a square tower, the faces of adjacent legs are parallel to each other. A ladder, put between two legs sees axial loading when/if one of the legs starts/tries to buckle in toward the adjacent leg- the compressive force is straight along its long axis. With the leg faces at 30 degree angle, that same beginning of buckling puts a non-axial load on; the force is at an angle, and it acts immediately to start bowing the ladder; starting the ladder buckling. So its buckling strength is MUCH lower than if it were under axial loading; need a much stiffer (i.e., much heavier) ladder to hold against the force. The other issue is getting square load block properly centered on the top leg end triangle- so that the center, the eyebolt, is centered between the legs. If its off at all, one leg gets disproportional loading, breaks early.

Balsa Man- perhaps you mid understood my question.
Understood that the bracing would not have much points of contact with the leg itself
my idea is to use 60 degree equilateral triangle leg sticks, described in my post above?
any thoughts?

I'm sorry, my bad; didn't read carefully before responding.
Yes, equilateral triangle cross section legs would... pretty much take away the issue of non-axial loading on ladders; adjoining leg faces would be parallel, ladders would have full contact with leg faces. I say 'pretty much', because I'm not sure what the direction(s) of buckling would be. In a square cross section, it'll be toward one of the faces (shortest cross section). In equilateral triangle, I'm not certain it would be. I'll do a bit more research and get back on that.
As I think you're noting/recognizing, diagonal bracing (Xs or Zs), however, would have essentially no contact area (just the edge). Under tension, virtually no joint strength, and no way to put on gussets (which, again, would have only the very edge of leg for glue. Three legs would save only a very little on weight, because density/BS would have to be upped significantly from 4 leg configuration. Basic density vs buckling strength relationship is double density, BS goes up about 2.25x, so it's in the .25 you gain a little. The other issue if getting precise 60 degree triangle cross section cut.

[Balsa Man]

Hey, glad we got things figured out- understand typos/etc.

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.

Hi Balsa Man,

Do you have any update on how well this method for the bottom of the tower works? I built my tower last night with on 4 tension strips on the bottom instead of ladders, and due to time constraints, I won't have time to actually test it and build another before the upcoming competition. But, after building, I'm having a little bit of doubt about if it will hold up, and am considering adding in ladders on the bottom. Since ladders are anchored flat against the leg, don't they prevent any movement of the legs that might result in buckling? I'm worried about buckling that might happen first on the lowest segment of the tower.

I'm having a little trouble following. The 'bottom band' tension strips go right down at the bottom, just above test base, so when you say 'built ...with...strips on the bottom instead of ladders', there is/should be no ladders right at the bottom; first ladder, going up, is....whatever bracing interval you're using...say 1/5, 10-ish cm above the base. The forces at the bottom of the legs have an outward component, so band at the bottom acts in tension to keep the ends from moving out. The ladders work only in compression (blocking inward buckling at the point they're located). With butt joint, virtually no strength to prevent outward bowing (that's what the Xs do). First ladder blocks buckling at the top of the lowest segment. The leg ends kicking out is not a buckling type of failure.

From what happened at regionals over the weekend, my read is if the lowest set of Xs are....put on fully tensioned, they'll hold full, or very close to full tower load. Its borderline. Bottom strap takes things from borderline to solid, but does add a little bit of weight.
Does this answer what you're trying to figure out?