Designs

[thsom]

Yup. The "right" density range is around that of 0.9gr/36 1/16th square, so at 1/64th, a quarter of that; ~2.25gr/36"

However, at normal density isn't 1/16 squared around .3-.5 grams?

you have to specify length... I think you mean 24'' though
Ya I mostly find .3-.5 with occasional .1 or .2 and a few dozen .6+

No I mean 36", I mean I'm guessing since you said a 1/8 squared is 1.3 to 1.5, shouldn't 1/16 squared be .325-.375 because it is 1/4 of the mass

[mrsteven]

I'm sorry, I meant 36 now that I'm looking at the order form haha

Ya you find alot of .3 and .4 mostly so. I have only had to go through about 300 sticks of 1/16 b/c its the easiest to find ones to suit the needs. I've had to go through ~700 1/8 and 1/16 X 1/18 to find a few dozen possible candidates for correct density and stiffness

speaking of which: What do you guys do with your extra wood? legit I have soooo much and nothing to do with them

[LKN]

thsom,

I have the same problem. Even though at the beginning of the season I went to buy balsa by the masses at my local hobby shop, I still have dimensions and densities that I bought but never used throughout the season. I usually stash them somewhere, but since I really got into jigged building this year I was able to use the old sticks to help create notched points on my jigs for chimney legs. Of course, you can go further and jig your bracing pattern as well with extra wood, but I didn't do that this year for towers and I doubt many people did (Balsa man and others have done this for bridge in the years past, look through the image gallery).

SLM,

Out of curiosity, what dimension/density balsa did you use for the chimney legs in the tower you posted way back? The one that tapered to 1x4cm at the top. I am thinking that the design could be optimized in the chimney by using 1/16x1/8 balsa to cause the chimney to take the load in a certain plane and brace against that place more efficiently and use less bracings (especially to support compression/buckling force in the chimney leg) in an attempt to save more weight on my tower. Any thoughts? Should non-square chimney legs be used in the first place? I would assume so, as long as you take into account their moments of inertia in both planes to effectively brace the chimney correctly.

[SLM]

...
Out of curiosity, what dimension/density balsa did you use for the chimney legs in the tower you posted way back? The one that tapered to 1x4cm at the top. I am thinking that the design could be optimized in the chimney by using 1/16x1/8 balsa to cause the chimney to take the load in a certain plane and brace against that place more efficiently and use less bracings (especially to support compression/buckling force in the chimney leg) in an attempt to save more weight on my tower. Any thoughts? Should non-square chimney legs be used in the first place? I would assume so, as long as you take into account their moments of inertia in both planes to effectively brace the chimney correctly.

We used 1/8 x 3/64 balsa, with a density of about 190 kg/m^3, for the main compression members in the chimney. The chimney was braced every 7 cm in the stronger direction and every 3.5 cm in the weaker direction. The bracings were 3/64 x 3/64 balsa with a density of about 160 kg/m^3.

For the base, especially if you are using a rectangular one, either rectangular or built-up section (like T or L) makes more sense as it would reduce the number of bracing intervals in the long (20 cm) direction. We used a T section for the legs which enabled us to brace the legs every 9 cm in the stronger direction and every 4.5 cm in the weaker direction. If, however, you are using a square base, it is best to use a symmetrical bracing pattern in all four sides.

Our tower was a bit over-designed because we wanted to be able to load/unload it multiple times. You might be able to reduce the density/size a bit more if you are okay with he tower breaking at or near 15 kg during the first loading cycle.

[LKN]

Thanks SLM, but I got to ask. Why do you give densities in kg/m and how do you get ahold of wood dimensions like 1/8x3/64? Specialized Balsa doesnt sell this, and I haven't heard of a shop that is that specific or small in wood cross sections.

[SLM]

Thanks SLM, but I got to ask. Why do you give densities in kg/m and how do you get ahold of wood dimensions like 1/8x3/64? Specialized Balsa doesnt sell this, and I haven't heard of a shop that is that specific or small in wood cross sections.

We cut, and trim, the wood to the specified size(s) ourselves. The density measurements are also done in house. Other than convenience, we have no specific reason for using the SI units.

[LKN]

Wow, that must be an awesome convenience for this event. Very impressive.

I did some number crunching, and here is what I got.

Each leg (1/8 x 3/64) had a density of .66g/36” stick.
This is equivalent to (1/8 x 1/16) .88g/36” stick
And is also equivalent to (3/32 x 3/32) .99g/36” stick

Bracings (3/64 x 3/64) have a density of .21g/36” stick
This is equivalent to (1/16 x 1/16) with a density of .37g/36” stick

----------------------------------------------------------------------------------

Tower chimney in context (assuming same exact chimney leg shape for scenarios)

Taking the moments of inertia of the rectangle cross section and breaking it into components to imagine that a chimney leg was made out of a square leg from one of the cross section dimensions:

So, if I am correct in assuming this, then in a 1/8 x 1/8 leg with the same constant density, bracing every side of the chimney at 7cm should be near equal in buckling strength.

A 3/64 x 3/64 leg with the above constant density would need to be braced every 3.5 cm on all four sides to be sufficient in buckling strength.

Does this seem correct? I re-checked your PDF to see if this reasoning makes sense, and wondering if there are any ideas about this?

[SLM]

Wow, that must be an awesome convenience for this event. Very impressive.

I did some number crunching, and here is what I got.

Each leg (1/8 x 3/64) had a density of .66g/36” stick.
This is equivalent to (1/8 x 1/16) .88g/36” stick
And is also equivalent to (3/32 x 3/32) .99g/36” stick

Bracings (3/64 x 3/64) have a density of .21g/36” stick
This is equivalent to (1/16 x 1/16) with a density of .37g/36” stick

----------------------------------------------------------------------------------

Tower chimney in context (assuming same exact chimney leg shape for scenarios)

Taking the moments of inertia of the rectangle cross section and breaking it into components to imagine that a chimney leg was made out of a square leg from one of the cross section dimensions:

So, if I am correct in assuming this, then in a 1/8 x 1/8 leg with the same constant density, bracing every side of the chimney at 7cm should be near equal in buckling strength.

A 3/64 x 3/64 leg with the above constant density would need to be braced every 3.5 cm on all four sides to be sufficient in buckling strength.

Does this seem correct? I re-checked your PDF to see if this reasoning makes sense, and wondering if there are any ideas about this?

If an optimally designed chimney has a 1/8” x 1/8” section braced every 7 cm, then if you use 3/64” x 3/64” section instead, braced every 3.5 cm, the chimney would definitely fail.

Member 1, with a 1/8” x 1/8” cross section has a moment of inertia of (1/12) (1/8) (1/8)^3 = 1/49152.
Member 2, with a 3/64 x 3/64 section has a moment of inertia of (1/12) (3/64) (3/64)^3 = 1/2485513.

Assuming that both members have the same density, then since the ratio of the two moments of inertia is approximately 50, then the square of the ratio of their unbraced lengths has to be 50 (based on Euler's Buckling Equation), if you want both members to have the same buckling strength. This means if member 1 is braced every 7 cm, Member 2 needs to be braced every 7/sqrt(50) = 1 cm.

To further clarify the point, here is another example.

Moment of inertia of a rectangular section is (width)(height)^3 / 12. If I use a 1/8” x 3/64” section for the chimney, then in the stronger direction (the direction with the bigger moment of inertia), I get:

(1/12)(3/64)(1/8)^3 = 1/131072

And, for the weaker direction, I get:

(1/12)(1/8)(3/64)^3 = 1/932068

Since the ratio of these two moments of inertia is about 7, then the ratio of the unbraced lengths needs to be sqrt(7) = 2.6. This means if I brace the member every 3.5 cm in the weaker direction, then the member has to braced every 3.5 x 2.6 = 9 cm (or less) in the stronger direction.

[T-B]

Thank you so much for giving that calculation. Thinking that we will really need to understand this for boom

[SLM]

Thank you so much for giving that calculation. Thinking that we will really need to understand this for boom

You are welcome.

The design of a boomilever is a bit different than towers since one needs to take into account bending moment as well as tension and compression in certain members of the structure. But, more on this later.