Designs

Balsa Man
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Re: Designs

Post by Balsa Man » March 23rd, 2011, 1:38 pm

lllazar wrote:Should the chimney be slightly angled inward so that the top of the chimney has a smaller square than the bottom? I feel this would help to eliminate balance issues.
Its not actually a "balance issue"- it is a stability one. If the sides are straight, if you have ANY movement of the load (as in bucket swing), you induce a tipping moment. If the sides slope in; let's say 2 degrees, load movement within 2 degrees does add load in the leg(s) it swings toward, but does not induce tipping moment until it gets outside 2 degrees. Just like with bridges last year- those with parallel sides, if the bucket swung across the span at all, over they went. Those w/ slide slope in of the sides, no problem. With the load 50 cm uup, the effect is even more important.
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Re: Designs

Post by iYOA » March 23rd, 2011, 1:46 pm

DaBalsa wrote:This may be kind of random, but I want to hear some opinions. After having been to many competitions...I too have noticed that many schools that usually don't do so well at this type of event are suddenly doing so well. I think this is because of the new scoring system. Although it is good in theory (more realistic), I think they should change it back to how it always was. I actually went to the NY State competition to watch the high schools teams compete in Towers and maybe get some ideas, and here I noticed the same thing. Also, I was watching FM/Spackenkill compete and FM's tower failed completely and I overheard that it had broken after impound, but had a mass of about 7? Maybe less? I couldn't get to taking a picture of it...but they got 41st place. I felt really bad for them, because if the rules hadn't changed, they would most likely have done much more decently. So essentially if a team that has worked really hard to develop a tower that could hold the entire load with a low mass had something bad happen to the tower (like FM), then the event would be a complete bomb because of the new scoring system. What say you?
i personally dont feel that this new scoring system is much different. if you look at the national results and compare the changes using the new and old scoring system, you will find that the general rankings dont actually change that much. There will be a few extreme exceptions for course but if a tower does well with the old scoring system, it will do well with the new scoring system. However, a stronger statement would be if a tower does well with the new scoring system it will definitely do well with the old scoring system. I noticed this with the towers that my partner and i built this year. Our old scoring efficiencies were much higher than our 2008 efficiencies
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Re: Designs

Post by lllazar » March 23rd, 2011, 1:48 pm

Sorry, that's pretty much what i meant by balance. But yes, i do remember doing this for bridge last year, it helped a lot with the bucket swaying. I suppose its the same for the tower. I actually made a new jig specifically for this today.
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Re: Designs

Post by sr243 » March 24th, 2011, 7:51 pm

I was wondering if the top part of the tower can hold 35 pounds and the bottom can also hold some 30+ pounds, should the entire tower combined hold about 30 pounds or more? Assume that the connection is basically perfect.

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Re: Designs

Post by soccerkid812 » March 24th, 2011, 8:25 pm

sr243 wrote:I was wondering if the top part of the tower can hold 35 pounds and the bottom can also hold some 30+ pounds, should the entire tower combined hold about 30 pounds or more? Assume that the connection is basically perfect.
15kg to 33 pounds
if both the top and bottom can hold the entire load separately, the whole structure will hold the load (assuming the connect is perfect, no tilting on the top, or any other construction problems)

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Re: Designs

Post by LKN » April 11th, 2011, 6:53 pm

Speaking of connecting the base to the chimney...

Do gussets play much role in tower building like they do in bridge building? I was thinking I would gusset the chimney to the base. But, on another thought, if the chimney is not attached perfectly straight it will end up failing at the connection point, and I wouldn't think a gusset could withstand that force. Even if the gusset could, the top of the tower would be leaning plus or minus a few degrees in one direction, causing it to fail. Am I correct in assuming this?

And on one of the two newly uploaded pictures in the Image Gallery, is the horizontal member between X bracings bearing any force? Or can it be removed to save weight?

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Re: Designs

Post by Balsa Man » April 12th, 2011, 4:18 am

Couple quick things-
If top carries 35 and bottom carries 30. 15kg is about 33 pounds. If the bottom only carries 30, then the tower will only carry 30.

Gussets- the force (from the load) is straight (or nearly straight) down onto the top of the bottom legs. If the geometry is right/precise- where the bottom of the top aligns precisely with the top of the bottom, and the ends are flat and in full contact, there is no force laterally, i,e., nothing that a gussett would brace against, or prevent. As things begin to distort, i.e., right before failure, you might get some, so, conceivable a little bit of help. Minimal if any, though.

The horizontal member in the Gallery. Dig pack through previous discussions on bracing. Like many subjects, it has been discussed at length before. Does it/do they carry any force?? The short answer is essentially very little, but what they do carry is critical. The rectangular base design means the bottom is acting as a bridge. The two parallel sides are trusses. The horizontal pieces joining them are often called ladders. Another subject to dig back previous posts to understand - and again, LOTS of discussion; what does bracing do? The legs (upper and lower are under compression loading. If the geometry is correct (everything is straight and aligned), that load runs straight through/along the leg. It is structurally speaking acting as a column. As you push doen on a column, at some point (some load) it will bow- "buckle"- called buckling failure. Bracing turns a long column into a series of stacked, shorter columns. Given the same material, a shorter column will have higher buckling strength. The bracing in question is bracing against the legs buckling in toward each other. If/as they try to bow in that direction, the ladder prevents that. The force is small at first; and it does not take much strength in the bracing to resist it. At some point, something has to give- unless something fails earlier, the braced leg(s) will bow- in the direction of least resistance. At that point, the ladder could, depending on the direction of bowing, see either compression load (legs bowing in toward each other, or tension load (legs trying to bow out, or other, more complex loading (legs bowing in another direction).
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Re: Designs

Post by hpfananu » April 12th, 2011, 7:54 pm

I think this question should go in designs but I'm not 100% sure...
Is there a big significance in using bracing on the outside (overlapping) versus bracing on the inside (butt joint with it cut to the correct angle)? (I think it's almost like a miter joint BUT instead of always 45 degrees it just usually fits the inside of the little part being braced. I've been using lap joints but my towers haven't been doing so well and my partner, who's been using butt joints, has been doing significantly better so I was wondering if I'm just not building correctly or do miter/butt joints do better?
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Re: Designs

Post by SLM » April 13th, 2011, 5:06 am

hpfananu wrote:I think this question should go in designs but I'm not 100% sure...
Is there a big significance in using bracing on the outside (overlapping) versus bracing on the inside (butt joint with it cut to the correct angle)? (I think it's almost like a miter joint BUT instead of always 45 degrees it just usually fits the inside of the little part being braced. I've been using lap joints but my towers haven't been doing so well and my partner, who's been using butt joints, has been doing significantly better so I was wondering if I'm just not building correctly or do miter/butt joints do better?
If the member is being compressed, then an (almost) perfectly constructed butt joint would be more effective, as it allows the (compressive) force to be transferred to the member by bearing action between the two members. Here, the glue takes no part in transferring the force (since the two members are in direct contact and pushing against each other). However, if the member is in tension, then glue plays a significant role in the load transfer. That is, for the force to be transfered from one member to the other, it has to go through the glue. For tension members, the larger the glue surface area the better (the less the glue is going to be stressed). This is where lap joint has an advantage over butt joint, since you have a better control over the glue surface area.

The density of the wood also plays a role here. A bracing that has a low density could fail easier at a lap joint than a butt joint.

We've had no difficulties using lap joints for the bracings throughout the tower. Whenever possible, we use lap joints since they are easier to construct.
Last edited by SLM on April 13th, 2011, 9:49 am, edited 2 times in total.

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Re: Designs

Post by Balsa Man » April 13th, 2011, 5:29 am

SLM is right-on.
I'd been working on a reply when his post popped up.
So, what he's saying, in a bit more detail:

“Form follows function…”
Depends on which part of the bracing, and the bracing sysytem. Again, it would be worth your time to dig back through prior posts- tower from this year, bridges from last year. There’s been a lot of good, detailed discussion.
The bracing is all about supporting a point in a column (the leg) – preventing it from moving when the leg is put under compression loading along it’s axis- cutting the effective column length down, and by that increasing the load at which it buckles.
For “Xs and ladders”, think about what each does, and how joints work.
Butt joints (without gussets) are inherently very weak in tension; lap joints are much stronger. The load a lap joint can carry depends on the glue area, and the density of the wood. Lighter (lower density) wood will strip- “shear” away more easily; heavier (higher density) much less easily.
When an axial compression load is put on a leg, at some point, it wants to/it will buckle. That buckling will take place in the direction of least resistance.
In an Xs and ladders system, the ladders act to prevent the leg from buckling inward, and the Xs act to prevent buckling outward. So, the ladders come under, and control a compression load- one or both legs pushing against the end(s). It takes just a tiny amount of glue to keep them in place; the forces are really acting to hold the joint together. Especially in the upper portion of a tower, the ladders are short, and can be of very light material (we’re using 3/32nd balsa at 0.6 and 0.5 gr/36”). That’s pretty soft stuff. If put on with a lap joint, as the legs try to bow inward, the forces are working to tear the joint apart; the joint will at some point fail in a “shear” mode, where the leg pulls away from the ladder. With a soft ladder, that doesn’t take much force. So, ladders work best in between the legs, butt-jointed.
The Xs act to keep the braced point from moving outward, so, when they’re working, they come under, are under, tension. As mentioned earlier, the strength of a joint in tension depends on the density. The lowest density wood in the joint will determine the strength. Since butt joints don’t work at all well in tension, a lap joint is needed. Reasonably high density is needed. Using square stock for Xs, when you get to high enough density for a sufficiently strong joint, you have WAY more tensile strength than you need. That is what has taken us in the direction of thin, high density strips for Xs. Take a look in the Gallery, under bridges last year- look at the bracing joining the two sides of the bridge. The Xs- tension strips we’re using on our towers are about 2mm wide, 1/64th thick. They’re stripped from a sheet of 36” x 3” x 1/64th weighing between 8 and 9 grams. They have a(tested) tensile strength around 5kg- way more than needed. They weigh very little. The trick/key for putting them on and having them work is to make sure they are tight- you have to pull a little tension as/while you glue them on. Attach one end. Then put glue where the other end joins; pull the strip along its axis, swing it into contact; hold than slight tension till the glue goes off (or have your partner give it a little shot of accelerator); trim off the end you were pilling on.
This system will get you very good rigidity in the overall structure, with both the joints and wood working…..for you, rather than against you. Our towers, using this approach – 3-leggers w/ 3/32nd bass legs – are running right at 8 grams.
Len Joeris
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