Designs

[LKN]

SLM,

Thank you for the other explanation PDF, it was great conceptually for our building techniques.

I remember an analysis that you did last year mentioned the difference between |/|/|/|/| and |/|\|/|\| that the second was more stable, but not necessarily improving strength against "perfectly" vertical failure. I have to ask, why did you go with the first design and not alternate the directions of your diagonal bracing in the chimney? I also recall last year you posted a tower that had diagonal Z bracing all going the same direction as well. The only reason I can think is simplicity when building the tower, but wouldn't the extra time (and at the caliber of competition your team builds to) to alternate the Z-bracings in the chimney be worth it? Looking forward to your response, thanks.

[SLM]

I remember an analysis that you did last year mentioned the difference between |/|/|/|/| and |/|\|/|\| that the second was more stable... I also recall last year you posted a tower that had diagonal Z bracing all going the same direction as well. The only reason I can think is simplicity when building the tower, but wouldn't the extra time (and at the caliber of competition your team builds to) to alternate the Z-bracings in the chimney be worth it?.

In general, bracing in a structure serves two purposes. It controls member buckling and it controls the overall stability and lateral movement of the structure.

For buckling, the bracing pattern per se is not too relevant. Of relevance is the ability of the bracings to effectively reduce the length of the compression members. For example, when we brace a long member at its midpoint, we reduce the member's length by half thereby increasing its buckling strength. We do not necessarily care what pattern of bracing is being used here, as long as the pattern effectively braces the member at its midpoint.

Bracing pattern plays a more central role for the overall stability and lateral movement of the structure. Although Z bracing uses the least amount of material (in terms of length), as you mentioned, it is not the most effective pattern with regard to controlling side-sway, compared with the other patterns. A tower with a Z pattern tends to side-sway more. The question is how much more? For a shorter tower, for us, the Z pattern posed no challenge last year; we benefited from its economical use of material without being adversely affected by it.

Last year, we did not get a chance to actually experiment with different bracing patterns. We used Z because of its simplicity and our prior experience. For a much taller tower, however, Z may not be a viable option. For this year's 70-cm tower our starting pattern is X (the most conservative one). We'll see if we can eventually use a less conservative pattern without causing the tower to fail prematurely.

[mrsteven]

I've actually seen a hand full of Z braced towers in IL that were 70 cm and holding the full weight at about 9 grams. Quite impressive.

[iTzDiamondFirexD]

Concerning X-Bracings on the chimney of a tower..

What would be a better technique on creating X-Bracings? This is my first time making an X-Braced Tower(Previous towers were Z-Braced).

and

OR

and

[thsom]

1st option

[havenguy]

In my opinion, I think that the 2nd option is the better one. It seems like there would already be tension in the first option since the wood is curved and you used force to change the natural shape of the wood. This would make a peice of wood break faster than one that there is not tension on. Also, our team has tried both and it seems like the 2nd option is easier to build and works better.

However, the first option may work if the two cross bracing are 1/16" and the legs are either 1/8" or 3/32". That way you could fit the 1/16" on the back part of the leg and the front part, thus not curving the wood. I think you should try to experiment, and see which one works better.

[mrsteven]

If I have a tower with many 1/16 X 1/16 intervoles for bracing, do I need to have a high density main compression member? Or because of the many Z style bracings I can have a lower density (stiff) member?
I know that more gluing surface area makes stronger bonds, so instead of using square wood on the base Xs I use a rectangular, would this be a low or a medium/high D? I've seen a few successful ones with rectangular members there that look somewhat large, and I'm wondering how they got ~9 grams with such members for Xs

Trying to get a tower thats 70 cm under 10 grams to hold all.

[Balsa Man]

Concerning X-Bracings on the chimney of a tower..

What would be a better technique on creating X-Bracings?

First, as I say each time I post something describing how we do it, I'm not saying this is the only, or absoulte best way to go; just that it works, it works well, and it is.....competitively light.

Second, on that basis, my answer to your question is neither

Third; there are at least half a dozen descriptions this year. Do yourself a favor and read back through posts- there are very good descriptions and discussions on the whole matter of bracing from many people who know what they're talking about.

Fourth- how we do it:
We're using 3/32nds bass for legs.
We use "ladders" and "Xs"- there are ladders- horizontal pieces- at the top and bottom of eaxh X'd section.
The ladders, which are butt-jointed on the inside faces of the legs; they fit in between the legs; are the same (cross-sectional) size. Ladders are 3/32nds balsa- pretty low density.
The Xs are 1/16th wide x 1/64th balsa strips Ppretty high density.

The ladders act to prevent buckling failure of the legs bowing in toward each other; when buckling in that plane/direction starts to occur, the ladders will come under compression; as long as it doesn't buckle, the leg will be prevented from buclking. Compression loads at the onset of buckling are quite low, so the ladders do not need to be very strong to work.
The Xs act to prevent buckling failure of the legs bowing out away from each other; when buckling in that plane/direction starts to occur, the X strips coming in to the joint from above and below will come into tension; as long as they don't have to straighten out (that is, if they don't have any slack in them), the leg will be prevented from buckling. This means these strips need to be put on without ANY slack- pulled out flat and straight. They are not glued at the point they cross.
To see what this looks like, go to the Gallery, under bridges (2009); look at the top of our bridge; imagine the two top truss members as a pair of legs- in the bridge, they are under compression loading, just as the legs of a tower are.

Hope this is helpful.

[mrsteven]

Concerning X-Bracings on the chimney of a tower..

What would be a better technique on creating X-Bracings?

First, as I say each time I post something describing how we do it, I'm not saying this is the only, or absoulte best way to go; just that it works, it works well, and it is.....competitively light.

Second, on that basis, my answer to your question is neither

Third; there are at least half a dozen descriptions this year. Do yourself a favor and read back through posts- there are very good descriptions and discussions on the whole matter of bracing from many people who know what they're talking about.

Fourth- how we do it:
We're using 3/32nds bass for legs.
We use "ladders" and "Xs"- there are ladders- horizontal pieces- at the top and bottom of eaxh X'd section.
The ladders, which are butt-jointed on the inside faces of the legs; they fit in between the legs; are the same (cross-sectional) size. Ladders are 3/32nds balsa- pretty low density.
The Xs are 1/16th wide x 1/64th balsa strips Ppretty high density.

The ladders act to prevent buckling failure of the legs bowing in toward each other; when buckling in that plane/direction starts to occur, the ladders will come under compression; as long as it doesn't buckle, the leg will be prevented from buclking. Compression loads at the onset of buckling are quite low, so the ladders do not need to be very strong to work.
The Xs act to prevent buckling failure of the legs bowing out away from each other; when buckling in that plane/direction starts to occur, the X strips coming in to the joint from above and below will come into tension; as long as they don't have to straighten out (that is, if they don't have any slack in them), the leg will be prevented from buckling. This means these strips need to be put on without ANY slack- pulled out flat and straight. They are not glued at the point they cross.
To see what this looks like, go to the Gallery, under bridges (2009); look at the top of our bridge; imagine the two top truss members as a pair of legs- in the bridge, they are under compression loading, just as the legs of a tower are.

Hope this is helpful.

Want to give some hints about mass of the sticks you use for each member type
I'm doing trials for the next few weeks with about 120 different density combinations and I want to know if I can rule out a few dozen combos before I start... a few dozen will save me immense amounts of time

[Balsa Man]

Want to give some hints about mass of the sticks you use for each member type
I'm doing trials for the next few weeks with about 120 different density combinations and I want to know if I can rule out a few dozen combos before I start... a few dozen will save me immense amounts of time

Some hints? Sure....
Leg density depends on what bracing interval you use. I've posted this before; real test data on column/buckling failure for 3/32nds bass- at an 8.5cm length for 1.4gr/24", failure about 7.5kg; for 1.9gr/24", failure at 11.5kg. You can build an inverse square relationship table to figure out the column strength at different lengths (and decide on a bracing interval from that. This density range covers 90-95% of the bass you're going to find; pieces at lower or higher density than this are quite rare. The median seems to be around 1.6gr/24"

Ladders- in the chimney - you can get 3/32nds balsa from around 3 point something gr/36" up (to much denser than you need); assuming a good precise build, where the ladders are just seeing coulumn bracing loading (as opposed to excess loads from a distorted structure) "pretty low density" is 0.5 to 0.6 gr/36". At 0.7gr/36" you're pushing into the overkill range.
Ladders in the base (for a C-tower). 1) the top ladder in the base is not just column bracing; it has to carry significant compression load, so it will need significantly greater density than chimney ladder braces 2) ladder/ladders (below the top) on the base legs are much longer than chimney ladders. Remenber that inverse square relationship in Euler's Buckling Theorem- twice as long means will carry 1/4 the load before buckling, so significantly heavier than chimney ladders
X-strips - for 1/16th wide 1/64th you start to get into a range that will (probably) work at a 3"x36" sheet around 7.5 -7.75 grams. Grain variations/imperfections can get you unless you have a good eye for areas to avoid; so a reasonable safety factor is +10 to 15%. Easy to get a feel for strength- cut some strips and pull; a kilo or two is plenty.
Have fun.