Scioly.org

Thsom, here are a few suggestions:
1. Try a design with more Xs, like 4 instead of 3. The more sections your base is broken up into, the stronger it should be.
2. Where is the base breaking? If the Xs are breaking but not the main pieces, try bracing with a larger cross section, or a higher density. If the main pieces are breaking, increasing the cross section and/or density should help.
3. Don't use horizontal pieces, except perhaps at the top and/or bottom of the base. I've found that they are not necessary in the middle.
4. Just as a tip, never rule out balsa or workmanship as a reason for premature failure. When I first started building balsa structures I focused only on designs. Then when I payed more attention to both balsa and workmanship, my scores really improved.

Hope this helps!

@BalsaMan

I recently ordered several sheets of the 1/64 inch balsa sheets and plan to cut them into the 1/16 inch cross sections as you suggested. Just a thought before I begin construction - how is 1/64 inches not so incredibly thin as to offer any support? I would assume that you would just use more x's or z's to account for the thin wood pieces?

guest382512345 wrote:@BalsaMan

I recently ordered several sheets of the 1/64 inch balsa sheets and plan to cut them into the 1/16 inch cross sections as you suggested. Just a thought before I begin construction - how is 1/64 inches not so incredibly thin as to offer any support? I would assume that you would just use more x's or z's to account for the thin wood pieces?

In the bracing system/approach I've described (ladders with Xs joining where the ladders meet the legs), the Xs are only working in tension- preventing the legs from moving/bowing in an outward direction. The ladders prevent moving/bowing in the inward direction- they're working in compression. The force both the Xs and the ladders has to be strong enough to resist is the beginning of bowing, which isn't very much- its only after bowing starts that the force builds up rapidly. If the Xs can hold a kilo; maybe a bit more, they should work.

The tensile strength of "high density" balsa is pretty amazing. A little real test data - 1/16th width, from a 3"x36" 1/64th sheet at 8.6grams, almost 2.1kg. The range of (3"x36") sheet weight that "should work" at 1/16th is from the low 7s up. Be aware that in a given sheet, some areas are stronger than others. If you hold a sheet up to the light, you'll see areas that are more and less opaque. Beware the areas where there is no apparent grain structure, and the light shines through the most. Easy to check if you're in the right range- cut a strip- glue one end to a little block of wood (an easy way to hold it)- glue the other end to something that weighs a couple pounds.

So, to get more strength, either increase the density of the Xs, or the width (e.g., twice as wide = twice the (tensile) strength. The number of Xs depends on the effective column length - the braced interval, between ladders that you need you need for the legs to support the load they see.
Last, be aware that it is very important for Xs used in this manner to be taught- i.e., placed with no slack. If there is slack, before they can provide tension bracing, they need to straighten out; that will allow movement in the leg, and that will lead to buckling failure.

Balsa Man,
What's the best way to make sure that tension members are taught?

Faustina wrote:Balsa Man,
What's the best way to make sure that tension members are taught?

I kinda thought someone might ask that. Over the years, I’ve tried to be very open on how-to details, and test data that could make a real difference. Usually, I wait till late in the season – like a week before State for us - so as not to help our competition too much. So, at this point, I’ll balance those two considerations by saying this. The strips need to be glued on the legs while they’re under a bit of tension- enough to pull them tight and straight- to take ALL the slack out. The hard way is 4 hands (2 people working together- using a strip longer than needed, 1 person pulling a bit on the ends, moving it into contact w/ the tower, the other person pressing at the contact points to get the glue to ‘go off”). The easier- 1 person – way is to make.....something that holds a tensioned piece that’s a bit longer than needed- think violin bow- glue it on in tension, then clip the ends outside the legs, then trim the excess.

Ah ok, thanks Balsaman! The last tower I built did not really have any effective compression members (I mainly used lap joints throughout). How crucial is having the members have wider dimensions (say a 3x32 squared over a 1x16 squared piece)? Having a piece with a width smaller than the main structural balsa sticks worries me, but I thought using the 3x32 squared sticks might be over-designing it.

for a chimney design I have been using:

I\I\I\I\I\I\I and I\I/I\I/I\I/I
(every 5 cm) (every 5cm)
I heard the second one was stronger or more stable so i used that but my question is if there is a way to change this design to make it lighter but still hold all 15kg?

I thought about doing this:
I/\/\/\I
(every 5 cm) will this one hold all?
or
I/\/\/\/\/\I
(every 3 cm) will this one hold all?

guest382512345 wrote:Ah ok, thanks Balsaman! The last tower I built did not really have any effective compression members (I mainly used lap joints throughout). How crucial is having the members have wider dimensions (say a 3x32 squared over a 1x16 squared piece)? Having a piece with a width smaller than the main structural balsa sticks worries me, but I thought using the 3x32 squared sticks might be over-designing it.

Sorry, but I’m not following what you’re asking.
A few thoughts that may or may not answer what you’re wondering about.

In the ladders and Xs system I’ve been describing, the ladders are the same cross section as the legs; put in between the legs, butt-jointed. In the chimney, the ladders are MUCH lower density than the legs. Smaller cross section (butt-jointed) ladders, at a higher density, could give comparable bracing, though.
In the base, where a leg mid-point ladder would be about twice as long as a chimney ladder, if its the same density as a chimney ladder, it will buckle at ¼ the load that would buckle a chimney ladder. That suggests a higher density would be needed.

Lap-jointed ladders will “work in compression”- i.e., they will resist two joined legs buckling in toward each other, but they’re not as efficient for 2 reasons. In a butt-joint under compression, the glue joint is a non-issue; no forces at work to take it apart. In a lap joint, you have a shearing force. With a low density ladder, it doesn’t take much force to shear the wood right under where it’s glued. Also, any compression force (from legs bowing in toward each other) is not axial (not along the axis of the ladder)- ANY force/movement inward will bow the ladder instantly; once that starts, buckling failure is right behind.

thsom wrote:for a chimney design I have been using:

I\I\I\I\I\I\I and I\I/I\I/I\I/I
(every 5 cm) (every 5cm)
I heard the second one was stronger or more stable so i used that but my question is if there is a way to change this design to make it lighter but still hold all 15kg?

I thought about doing this:
I/\/\/\I
(every 5 cm) will this one hold all?
or
I/\/\/\/\/\I
(every 3 cm) will this one hold all?

There are way too many other factors at work (with no information about them) to be able to say anything about the answers to your questions.... Also not at all sure what your drawings represent. I assume we're looking at a chimney on its side, with the vertical lines representing ladders (horizontal braces), and the slanted lines representing Z-braces. So, the first pair of figures is ladders every 5cm w/ Z-braces between; the second figure is (?) ladders every 30cm, w/ Z-braces every 5cm in between?, or ladders every 5cm, with 6 Z-braces between??

Balsa Man wrote:

thsom wrote:for a chimney design I have been using:

I\I\I\I\I\I\I and I\I/I\I/I\I/I
(every 5 cm) (every 5cm)
I heard the second one was stronger or more stable so i used that but my question is if there is a way to change this design to make it lighter but still hold all 15kg?

I thought about doing this:
I/\/\/\I
(every 5 cm) will this one hold all?
or
I/\/\/\/\/\I
(every 3 cm) will this one hold all?

There are way too many other factors at work (with no information about them) to be able to say anything about the answers to your questions.... Also not at all sure what your drawings represent. I assume we're looking at a chimney on its side, with the vertical lines representing ladders (horizontal braces), and the slanted lines representing Z-braces. So, the first pair of figures is ladders every 5cm w/ Z-braces between; the second figure is (?) ladders every 30cm, w/ Z-braces every 5cm in between?, or ladders every 5cm, with 6 Z-braces between??

Okay here is what i meant: This is what I am using:
I\I/I\I/I\I/I. Diagonals that span 5 cm and ladders that occur every 5 cm

Possible new lighter designs:
I thought about doing this:
I/\/\/\I. diagonals that span 5 cm with no ladders except at bottom and top
or
I/\/\/\/\/\I. diagonals that span 3 cm with no ladders except at bottom and top
and my question is if these two above will hold, if the one with ev. 3 will and if the one with ev. 5 will. and also, if you have any other designs that have been light and have held all 15 kg, lighter than the one i am currently using

OH and about the other factors. I am using good stiff but light weight pieces(.4 grams for 1/16th squared and 1.6 grams for 1/8th squared) and everything is little glue (all that's needed) and all of it is good workmanship with good straight tower and such, any other factors you need me to specify about?

OK, understand the drawings, and what you're wondering.

You'd said earlier (when asking about the base) that your chimney (with ladder/diagonals every 5cm) holds 15kg. I have no experience on the effectiveness of just diagonals. Testing is the only way I know to find out if it'll work. Obviously, diagonals every 3cm will a) be heavier than at 5cm, and/but b) provide some additional bracing (i.e., legs will carry something more than at 5cm)- I have no idea how much .

I gather from what you say, you're doing both ladders and diagonals in 0.4gr (per 36"stick, I assume), 1/16th x1/16th, which means they're providing both compression and tension bracing. Even very good, stiff 0.4 is pretty floppy, with very little tensile strength. Its just a guess - a gut sense - that you're pretty close to the (wood strength) limit (in your bracing) in your current configuration, and that neither dropping of all ladders except at top and bottom (using 0.4) will work. I could certainly be wrong.

Be very interested to hear how your testing goes.