## Designs

thsom
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### Re: Designs

Will bracing a chimney every three centimeters like this: \/\/\/\/\/\ be stronger than bracing a chimney every 5 centimeters like this: \l/l\l/l\l/l\l/l\l (like tower is on it's side). So, every three without ladders or every 5 with ladders.

flyingwatermelon
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### Re: Designs

Will bracing a chimney every three centimeters like this: \/\/\/\/\/\ be stronger than bracing a chimney every 5 centimeters like this: \l/l\l/l\l/l\l/l\l (like tower is on it's side). So, every three without ladders or every 5 with ladders.
I think you're overdesigning. Try five maybe six centimeters for the first design, and horizontal braces are not TOO helpful, but experimentation always is the best.

SLM
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### Re: Designs

thsom wrote:Will bracing a chimney every three centimeters like this: \/\/\/\/\/\ be stronger than bracing a chimney every 5 centimeters like this: \l/l\l/l\l/l\l/l\l (like tower is on it's side). So, every three without ladders or every 5 with ladders.
My vote goes to the second pattern, especially if you increase the interval from 5 cm to 6 cm. That way, either pattern braces the compression member the same way; the member is divided into 18 un-braced segments each having a length of 3-cm (assuming a 54-cm-tall chimney). The horizontal bracings are much more effective in controlling buckling than the diagonal ones.

Your bracing patterns are not necessarily over-designed. That is a function of the size and density of the piece you are using for the chimney. I suspect for 1/8" x 1/8" medium density balsa, the design is about right, although you still might be able to tweak it a bit more depending on the size/density you are using for the bracings.

thsom
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### Re: Designs

SLM wrote:
thsom wrote:Will bracing a chimney every three centimeters like this: \/\/\/\/\/\ be stronger than bracing a chimney every 5 centimeters like this: \l/l\l/l\l/l\l/l\l (like tower is on it's side). So, every three without ladders or every 5 with ladders.
My vote goes to the second pattern, especially if you increase the interval from 5 cm to 6 cm. That way, either pattern braces the compression member the same way; the member is divided into 18 un-braced segments each having a length of 3-cm (assuming a 54-cm-tall chimney). The horizontal bracings are much more effective in controlling buckling than the diagonal ones.

Your bracing patterns are not necessarily over-designed. That is a function of the size and density of the piece you are using for the chimney. I suspect for 1/8" x 1/8" medium density balsa, the design is about right, although you still might be able to tweak it a bit more depending on the size/density you are using for the bracings.
Thanks! So what I'm hearing is that the second design is stronger than the first one? I have them light enough and that doesn't matter. However, the second one is stronger. Efficiency isn't the matter because of how light the tower already is. So, based solely on strength, the second design is stronger, correct?

SLM
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### Re: Designs

thsom wrote:
SLM wrote:
thsom wrote:Will bracing a chimney every three centimeters like this: \/\/\/\/\/\ be stronger than bracing a chimney every 5 centimeters like this: \l/l\l/l\l/l\l/l\l (like tower is on it's side). So, every three without ladders or every 5 with ladders.
My vote goes to the second pattern, especially if you increase the interval from 5 cm to 6 cm. That way, either pattern braces the compression member the same way; the member is divided into 18 un-braced segments each having a length of 3-cm (assuming a 54-cm-tall chimney). The horizontal bracings are much more effective in controlling buckling than the diagonal ones.

Your bracing patterns are not necessarily over-designed. That is a function of the size and density of the piece you are using for the chimney. I suspect for 1/8" x 1/8" medium density balsa, the design is about right, although you still might be able to tweak it a bit more depending on the size/density you are using for the bracings.
Thanks! So what I'm hearing is that the second design is stronger than the first one? I have them light enough and that doesn't matter. However, the second one is stronger. Efficiency isn't the matter because of how light the tower already is. So, based solely on strength, the second design is stronger, correct?
A structure could fail in different ways, a member could buckle, the tower could have excessive side-sway leading to large stresses at the base of the chimney, a tension member could be too weak, a bracing could have inadequate strength, ... So, the term "strength" needs to be used in reference to the specific type of failure. From the standpoint of member buckling, yes, I do think the bracing pattern with horizontal members is stronger than the other pattern (based on the proposed spacing). With respect to side-sway(lateral displacement of the chimney), both patterns should offer more or less the same strength, they both resist equally lateral movement and distortion of the chimney.

Julian
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### Re: Designs

Would anyone be willing to entertain a discussion about tapered chimneys again?

I have just about finished building a new tower, which I hope to test in the next few days if my schedule allows it. All of our towers have been pretty standard so far, (i.e. 1/8" base, 3/32" chimney, with 1/16" Z bracing), but this time I introduced a taper into the chimney. In all my previous designs, I have used a straight, 5x5cm cross-section all the way to the top. My coaches and I noticed though that with this design, there was very little room for error, because even a slight deviation above 5cm would offset the loading block from the top of the legs. We tried to remedy this by placing a flat 1/16"x1/8" ring on top of the legs, but I think it has only been marginally beneficial. In my last test, my 9 gram tower held 13kg before breaking, and the breakage occurred in the top 1/4 section of the chimney. We are not able to determine if the break was due to a imperfect setting of the loading block or just the weakness of the wood, but we elected to go with a smaller cross-section anyway, to guarantee that the block would sit on all four legs in the future.

In my latest design, I have a 5cm chimney base taper evenly on all sides to a 4cm square at the top. Hardly a huge change by any means, but it is enough to solve a reoccurring problem, and it might even help a bit to offset tower sway during loading. However, despite having read the previous discussions about tapering (1,2,3), I keep worrying that all the horizontals are in compression. I know in SLM's excellent graphics, he shows that the top members undergo compression and the bottom members undergo tension, but that is assuming that the ends of the member are not fixed. In the chimney, for example, the bottom legs are locked into placed because it is connected to the base. Therefore, all the horizontals should undergo compression forces. Is this reasoning correct?

Even if SLM is right though, then it would make sense for one to vary the density of the horizontals depending on their placement in the chimney. We are currently using 1/16" bracing at 0.3g / 36" every 7 cm. Any predictions as to whether this is too little? We have found that 0.4-0.5g/36" is strong enough to hold up 15kg if built correctly, but 0.3g/36" might be pushing it. Perhaps in a tapered chimney, electing to use 0.5g/36" in the top half and 0.3g/36" in the bottom half would be the most ideal? I can't seem to think of any other way to get the tower weight down to a competitive level.
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Balsa Man
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### Re: Designs

Sure...
You raise some good questions. Working through from the top....
YES, there is an overall structural stability advantage with a bit of taper in the chimney; pretty much as you perceive it- a bit of latitude/safety factor under.....non-axial loading. A question, you say a straight-sided chimney at 5cm x 5cm; that means that at the base, you're significantly inside the 8cm circle at the bottom/top of base; the square that will fit in an 8cm circle is larger than 5x5.... That means your base legs are leaning in further than they have to, and with greater angle of lean, are carrying greater force than if less angled. You can get close to a cm of lean-in and a top that's only about 1/6th" in from 5cm x5cm.

A key when running a top smaller than the load block is a reliable, easy to do way of centering the block. You can eyeball it, but it doesn't take being off by much, particularly when you're pushing things to get those last few tenths of a gram off, to have one leg enough overloaded to reduce performance. A way that helps is two little, we call them block fingers. Little, very light guides to locate two opposite corners so the block is truely centered. We use 1/16th- 0.3-0.2 gr/36" if you have it; put on with just a tiny bit of glue. On ours, they are like 1/8th long. To set them up, draw a 5x5 square with diagonals. Invert the chimney; out it on the square so that the legs are on the diagonals. If the square is accurate, and your top is square, it will be centered. What you want is at two opposite corners, to have the ends of these fingers right at the corner of the block- your 5x5. Initially put them on so they're long enough to extend apst (and through) the corner point. Trim with razor blade. Yeah, they're delicate, but they weigh very little. They provide a way where both your fingers and your eyes can tell when the block is centered.

You mention 1/8th base legs and 3/32nds chimney leg. Transfering load at an angle through two different cross sections has its own set of issues/problems. I assume you've got testing that says you need to go that route, and that its lighter than you can get w/ 3/32nds all the way, but I wouldn't want to have to deal with it...
9gr to 13kg is quite respectable. Understanding the initial failure mode is key to identifying a smart way to fix it- to get better performance. This is a situation where a safety tower will tell you exactly what is going on, then you will know, and that will tell you the direction you need to be working in. How to, and pictures are here in this year's discussion, and archives/gallery. That the failure is (apparently consistently?) in the top quarter, suggests the possibility the leg specs(x-sect/density/bracing interval) may be ok for more weight, and the problem is in the bracing(or the glue joints of the bracing) near the top.

That rolls into the question of whether 0.3 1/16th is enough. Personal opinion, probably not; marginal for ladder/compression; really questionable for diagonal- tension. 0.3 has very little tensile strength, and is soft enough to invite shear failure where its glued to the leg, and using square stock for braces working in tension is wasting wood- it is not the entire cross section that's working for you. Higher density thin strips will get you the tensile strength 9in brace and in glue joint) you need at less weight...

Last, whether the chimney leg taper means compression loading on the ladders (other than at top and bottom); no, it doesn't.
Assuming the legs are straight, they carry the load straight down, axially. The ladders along the way only see compression when, and to the extent that, buckling tries to start. Yes, at the bottom, there is some force outward away from the center; yes, at the top, there is some force inward toward the center. Down in the base, these forces are significant (and need to be braced against). But they are not at work in between. The amount to which a downward force increases when put through an angled member (leg) is a function of 1 over the cosine of the slope angle. if you work up/look at a table of inverse cosines by angle, you will see that for the first few (5+) degrees, the change is out in the 3rd or 4th decimal place. In the base, the 3.75kg vertical load per leg (15/4) is 4.11 kg (at an angle of about 24 degrees). That by the way means that the ladder at the top of the base is seeing - just pulling from memory, pushing 2kg. So, there is a tiny bit of compression on a ladder right at the top, and a tiny bit of tension at the bottom, but as you note, the bottoms of the chimney are glued to/locked in to the top of the base legs, and they're braced by the top of base laddrers. Hope this helps
Len Joeris
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SLM
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### Re: Designs

Julian wrote:... My coaches and I noticed though that with this design, there was very little room for error, because even a slight deviation above 5cm would offset the loading block from the top of the legs.
I am not sure what you mean by "deviation above 5 cm." But, generally, the closer the top four corners of the chimney to the chain, the less negative impact unbalanced loading would have on the tower. Of course, there is a limit to how close you can get the corners to the centerline of the tower, but I don't see how your 4cm x 4cm top is posing more problem than your 5cm x 5 cm top.
Julian wrote: However, despite having read the previous discussions about tapering ... I keep worrying that all the horizontals are in compression. I know in SLM's excellent graphics, he shows that the top members undergo compression and the bottom members undergo tension, but that is assuming that the ends of the member are not fixed. In the chimney, for example, the bottom legs are locked into placed because it is connected to the base. Therefore, all the horizontals should undergo compression forces. Is this reasoning correct?
Your analysis is outstanding and your conclusion is correct. The reason the horizontal member at the bottom of the base undergoes tension is what you stated; the bottom of the leg is not fixed in place. It is a completely different story at the bottom of the chimney.

In the chimney, there are two places that I would not refer to the horizontal members as bracings. The very top and, more significantly, the very bottom members. At the bottom of the chimney, the horizontal members are indeed force-carrying members. They all carry a significant compression force. You should not treat them as regular bracings. They are not there to prevent buckling; they are there to carry a compression force proportional to the inclination angle of the legs.

Julian wrote: Even if SLM is right though, then it would make sense for one to vary the density of the horizontals depending on their placement in the chimney...
I would not use different bracings for the top and bottom halves of the chimney. I would only strengthen the very bottom and, perhaps, the very top horizontal members.

Vizard007
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### Re: Designs

Hey guys, I was wondering... do you think it'd be better to have the chimney completely perpendicular to the test base? Or do you think it would be more efficient to have the chimney slightly slanted in towards the center. Orrrr, do you think it'd make a difference not worth the effort? Thanks for the help!
It's so easy, even a Badger could do it.

Julian
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### Re: Designs

Vizard007 wrote:Hey guys, I was wondering... do you think it'd be better to have the chimney completely perpendicular to the test base? Or do you think it would be more efficient to have the chimney slightly slanted in towards the center. Orrrr, do you think it'd make a difference not worth the effort? Thanks for the help!
This was discussed in the posts right above you. Some useful posts to read are 1,2,3.

Thank you Balsa Man and SLM for your great insight as always. The tower I described in my previous post was completed over the weekend and weighed in at an incredibly light 6.47g, held 11.3kg, registering a respectable score of 113.5 at the Regional level. As you predicted, it looked like the failure was caused by the shearing of a particularly weak 1/16" square horizontal in the chimney. I was surprised how light the tower ended up being, but in hindsight, I would have gladly switched to 0.4-0.5g per 36" and traded the 1g increase to hold 3-4kg more.

I learned a couple things from this latest build, which I'll reiterate here. Any thoughts?
• 1/16" @ 0.3g/36" is pretty unreliable for bracing. As I was cutting sticks to make into horizontals, I noticed that the wood crushed very easily if it was cut straight up and down, and rather needed to be "sliced" to have a clean end. I understand now that 1/16" square pieces may not be the best dimension for bracing, but it is one of the most commonly stocked dimensions at most hobby stores. If you're going to go with it, make sure you use a slightly higher weight per stick.
• I know that Balsa Man has brought up repeatedly the effectiveness of high-density thin strips to use as bracing and I think this is the direction I will be moving to with the next design. Admittedly, I have been reluctant to make the leap, mostly because I didn't realize that you could specify the weight on SpecializedBalsa for any dimensions other than the ones listed here. Also, something like 1/32" x 1/16" is rather expensive per stick, and unfortunately I don't have the luxury to choose the wood on site like the CO-based teams. However, as the state and national tournaments inch closer, it seems that this is the next logical step to take.
• I guess a slightly cheaper method would be to strip my own wood for the bracing. No one on my team, past or present, has any experience with this though, so I have not tried it out. I'll probably have to go out a buy a balsa stripper before attempting to do this. At the moment, I don't trust I have the manual dexterity to strip wood with a straight edge and a razor blade. It is very encouraging that other members have posted about their experiences and tips on how to strip wood properly.
• The other major change is maintaining a close to 5x5cm square at the top of the chimney while still tapering the sides. I have known that the biggest square that can fit inside a 8cm diameter hole is about 5.6cm per side, but I disregarded that fact because it was much easier just to build a straight 5x5cm chimney. Balsa Man mentioned that you could get a whole centimeter of taper on the chimney, but I can't make the calculation work. If you start with the assumption that a 5.6cm square is the largest you can have, then won't you only be able to make a .6 cm taper to the top? If I'm not mistaken, the only way to get a 1cm taper is going smaller than 5cm at the top (e.g. 4.5cm square), or starting the chimney further down from the 8cm hole (i.e. taper from 6cm to 5cm square).
• The other critique was about switching from a 3/32" cross-section to a 1/8" cross-section. To be honest, I'm not quite sure why I do this, other than that it has worked well in all my previous towers. The densities of the wood are both the same, but I've always figured that using 1/8" legs on the base would give it a bit more surface area on the ground, though I'm not sure that translate into any measurable difference in carrying capacity. I have been interested to try using the angle-iron lamination with a soft balsa core, but the task is rather daunting without any previous experience and not a whole lot of time left for experimenting. I do believe that if I can fix all the previous issues though, this option will eventually be explored, hopefully in time for the national tournament.
One thing that has stopped me from fully committing to high-density, thin-strip bracing for the chimney has to do with something Balsa Man has mentioned a couple times previously. It's about a technique his students use to glue the bracing with tension already on the wood member. Despite rereading old posts, I have not been able to come up with a reasonable way to do this solo. You hinted that something like a violin bow could induce a tension while gluing it into place, but I was not able to create something like that with the tools at my disposal. Also, I'm guessing you would have to glue in all the diagonals before the horizontals, because it would be very hard to place diagonals flush with the horizontals using the method described above.

I know it's still a bit early in the season, but would you be willing to offer up another description of how to go about doing this? I think that is the key to getting the most use out of the HDTS in Towers.
Last edited by Julian on March 14th, 2012, 9:23 am, edited 1 time in total.
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