Designs

[Freyssenet]

What would you say is the reference density of balsa? From a bit of research, i got numbers around 150kg/m^3, does that sound about right?

We recently bought several medium and heavy density balsa sheets from SpecializedBalsa.com. The density of the sheets labeled "medium" seems to be around 250 kg/m^3. For the "heavy" sheets, we are getting numbers around 400 kg/m^3.

The specific densities of these sheets are 0.25 and 0.4 respectively. such densities seem high for balsa, but not high enough for basswood. Can you re-check? I would have expected 150 kg/m^3 for medium density balsa indeed (or a specific density of 0.15).

[lllazar]

Yes, typical "contest grade" balsa (which is really light and doesn't need to be as stiff/dense as structure balsa, mainly used in free flight) comes to be around 100 kg/m^3, and i wouldn't think that balsa used for structure building would be too much heavier than that, certainly not over 200...maybe you just got a heavy order?

Also, what is meant by the terms "soft", "medium" and "hard", it comes up a lot on quite a few online stores such as this one:

http://www.balsawoodinc.com/balsawoodproducts

Im wondering if they are referring to the stiffness, because they do in fact say that when they hand pick based on the above criteria of "soft" - "hard", they don't mean density. It would make sense because the really floppy pieces i've seen after testing dozens of sticks for stiffness - well, i can crush them by squeezing with very little force. Perhaps this is soft? On the other hand, stiff balsa tends to be "hard", as in i can't crush it, its sturdy and not floppy (obviously).

[jander14indoor]

Experienced balsa pickers use feel or stiffness to quickly select wood. In general, soft balsa is light, hard is heavy, but as already discussed this is only a general trend. For SO structures (and indoor free flight) at competitive levels you must take this the next step and select for BOTH density and stiffness. That's why so much wood is simply thrown away or used in jigs or other secondary purposes. Its simply junk wood, heavy AND soft, where you want the best wood, light AND stiff.

Jeff Anderson
Livonia, MI

[JimY]

Each curve represents all the acceptable (un-braced length, member size) pairs that result in the 35-cm-long member having the same weight. For example, using the bottom curve, I see the following possible choices for the member:
Size: 1/32” (0.793 mm), maximum un-braced length: ~30 mm
Size: 1/16” (1.587 mm), un-braced length: ~ 45 mm
Size: 3/32” (2.381 mm), un-braced length: ~ 58 mm
Size: 1/8” (3.175 mm), un-braced length: ~ 65 mm
Size: 3/16” (4.762 mm), un-braced length: ~80 mm
size: 1/4” (6.35 mm), un-braced length: ~90 mm
.
.
.
Each of these choices prevents member buckling and results in the entire member having a weight of 0.4 grams. However, keep in mind that buckling is not the only mode of failure here. The member could fail due to the lack of adequate compressive strength.

Also, these charts and equations cannot be used blindly, you need to be aware of their underlying assumptions and limitations.

As far as material shape/dimensions, another possiblity that should not be brushed aside is to use L shaped compression beams. For example, a 1/8" x 1/8" cross section and an L that is 5 mm on a side and 1 mm thick has almost the same cross sectional area (the L has a bit less). Yet the L shape with the same material will always be stiffer than the square piece since bh^3 is higher for the L than the square by a substanial factor. Making great L joints is not the simplest thing to do, but all three teams that I coach in this event do them successfully. Why do it? It allows you to be less at the mercy of the balsa. Even the glue joint between the two pieces that are used to make the L adds to the stiffness and probably the compressive strength as well.

I've seen very few teams even at the national level using L shaped girders for the stuctural building events. Too bad for those that don't used them. As much work as some teams put into these events, to me its a no-brainer to at least try the same truss design with both square and L girders and see how each fares against the scoring formula. I put a post into these pages about once a season touting L shaped girders. The results speak for themselves, with 4 national B division wins and two other national medals in B division bridge events, all with L girders.

Then going back to the charts and discussion, we've built and tested 2 full size towers now, one for B and one for C. The thing I wanted to mention was how each failed. Both are rectangular designs that use L shaped girders with plenty of Xs between the girders in both directions (the actual number is a trade secret). Failure in both cases was not due to the girders flexing and breaking between the Xs but rather the girders in the narrow direction pinched together in the lower section of each tower to the point that gross girder bucking occurs. The girders get pinched together, as evidenced by the Xs that connect them bowing more outward as more load is applied. In addition, when they visably move in the outward direction, this is the sign that failure is only a couple seconds away and that even if you stop loading it, it will still fail. The easy conclusion from our tests was that the Xs between the girders were too weak to handle the beam's compressive loading in that portion of the tower. The first tower tested was for C division and only had Xs between the girders. Then for the B tower, we corrected by adding horizontal stiffener pieces between the Xs in this fashion: XIXIXIX. We used two different materials for this, one on each side of the lower section, to see if we could tell the difference as it was being loaded. Indeed, it was rather easy. On the side with the wimpier horizontal stiffeners, the Xs started bowing outward well before they started bowing outward on the side with the stronger horizontal stiffeners. Failure occurred on the side with the wimpier horizontal stiffener material.

So, while the theoretical discussion is great, you really have to get in there an very closely watch what's going on as the device is being loaded, be it bridge, elevated bridge, boomilever, or tower. If you can spot what's going on, many times you can make corrections that add only a minor amout of mass to the device, yet can more than double your score. This is a much faster, easier, and far cheaper approach to these events than what I've seen in these threads over the years with all the discussion on balsa density, stiffness, etc. Of course, you can do what you want. But,if you really want to improve, challenge and change those old paradigms!

[baker]

Are you saying the 'L' is 5 mm on one side and 6 mm on the other side? |_ If this is true, doesn't the amount of glue required to assemble the 'L' become a substantial? Are you using CA glue or something else like Duco or Wood Glue?

[lllazar]

Thank's Mr. Anderson, i think that specifying the stiffness id like would cut down a lot of selection time, i could then sort out the light "stiff" pieces, and set aside the overly dense strips.

[JimY]

Are you saying the 'L' is 5 mm on one side and 6 mm on the other side? |_ If this is true, doesn't the amount of glue required to assemble the 'L' become a substantial? Are you using CA glue or something else like Duco or Wood Glue?

Sort of, but I like to make the dimensions the same for each leg of the L. So, if you are using the same cross sectional area as a 1/8" x 1/8" beam, then you can use 10 mm of 1 mm thick sheet material (usually sold as 1/32" thick material). Then, the 10 mm would be cut into a 5.5 mm strip and a 4.5 mm strip such that when assembled, the legs are each 5.5 mm. As far as the glue required to make the Ls, it does take several drops to do these beams for each structure. For towers, it's probably more than 50% of the total glue used, but it still isn't much if you know how much to use and how much is too much. We exclusively use Odorless CA with gap filling viscosity, which has a black cap. We've been using this for about 10 years now and see nothing out there that is worth changing to.

[SLM]

As far as material shape/dimensions, another possiblity that should not be brushed aside is to use L shaped compression beams...

Indeed, for compression members there are more efficient sections than a square or a rectangular one. Angle (L) is one. One can also use T or I shaped sections to improve the various buckling strengths of the member. Among these, angles provide most resistance against lateral buckling (since, as JimY mentioned, comparatively, they end-up having the largest moment of inertia). However, all these sections, since they have thin partially supported plates (think of the legs of an angle as long thin plates that are free along one edge and only supported along the other edge) are rather weak with respect to local buckling (the buckling of only part of the member); they have to be properly braced for that.

Besides L-shaped built-up sections, there are other alternatives that could/should be explored, especially for the top part of the tower. I particularly like the one(s) that eliminate the need for any cross bracings!

[Jo_squirrel]

I don't know if this question has been asked but I had a question regarding the 20 x 20 square. Is it a violation if no actual piece of wood is touching the space within the square? For instance, would it be a violation if I were to make a triangular base and had the three legs touching outside the square with braces connecting the legs to each other? (The braces would then inevitably fall within the square but not on the table)

I hope I didn't phrase my question in too confusing a manner...

[Freyssenet]

I don't know if this question has been asked but I had a question regarding the 20 x 20 square. Is it a violation if no actual piece of wood is touching the space within the square? For instance, would it be a violation if I were to make a triangular base and had the three legs touching outside the square with braces connecting the legs to each other? (The braces would then inevitably fall within the square but not on the table)

I hope I didn't phrase my question in too confusing a manner...

Jo, I would glue the horizontal stick tying the three legs at 1 mm above the base. In my opinion, this would remove any ambiguity when judging compliance of rule C that states: no portion of the tower is allowed to extend below the top surface of the test base prior testing. If the horizontal sticks are touching the test base surface, an event captain may judge negatively your structure under the argument that, before testing, the horizontal sticks are sagging a little under their own weight and that at and around their mid-length, the sticks are slightly under the test base surface. By gluing them to the legs a tiny amount above the test surface you will make sure your tower will be compliant.