Elevated Bridge B/C

[matthew8817]

I thought i remember seeing something about no commercially crafted materials. I'm not sure if that has anything to do with coffee stirrers, but still.

[Littleboy]

Aren't coffee stirrers plastic? Don't risk it, balsa has a better efficency.

[Balsa Man]

Are coffee stirrers considered "wood products" by the rules? I want to construct a bridge out of them and dowls (they are both strong and light). Please let me know- I have only seen balsa and basswood. Are coffe stirrers and dowls illegal?

From Scioly Rules Clarification/FAQs:

(section: 3 / paragraph: i / line: 2)

01/09/2010 - 21:05 Can you clarify what exactly is considered a wood product?
Wood is a tough, fibrous material from the branches and trunks of trees and shrubs, usually cut to some useful size and dried, but still recognizably the material cut from a tree. "Wood products" are materials made from wood, with the wood fundamentally altered from its natural state, typically commercially available and made in a manner not readily produced by the students. Paper and particleboard, masonite, and similar materials have no distinguishable remaining grain structure and exist solely as a man-made material. Commercially made plywood is still recognizably made of wood fibers, but the process of producing this material is not performed by the student. Wood which has simply been cut into smaller pieces, and dried (like toothpicks), do not fit this definition of "wood products". Students may produce their own plywood by laminating pieces of wood. On the other hand, student-made particle board, putty, or paper, in which the underlying structure of wood fiber is destroyed, can no longer be considered to be "wood"; it is a wood product regardless of who made it.


So, plastic coffee stirrers are neither wood nor wood product. All the "wooden" ones I've seen are just pieces of wood- as in ok. Dowels would also be ok per above.

Now, why you would want to use them is another question.... not for max efficiency, for sure

[nejanimb]

A girl came up to me once after a regional competition and thought I had been so clever for building my bridge out of toothpicks. Of course, we hadn't, but it was sort of funny that people thought that's what it looked like.

Never have tried dowels, not even balsa ones, since I just figured they'd be too difficult to make joints with. I did try triangle shaped pieces at one point though... those were certainly very interesting, and I could see them having their place with certain designs.

[Balsa Man]

How does this look for cross bracing?


and if you can't see it go here
There is one full-length piece and the gusset helps to hold the other two pieces.
5 pieces in all 2 gussets 1/32 X 1/16, 1 single piece 1/16 X 1/6, and 2 small pieces 1/16 X 1/16

Going back to this, you apparently have access to 1/32x1/16 balsa. That's all I used. I just had some pieces going straight across and then for my x-bracing, you can just press one on top of the other. Each one will only be deflected ~1/64" so they'll be fine.

Just a quick observation - going back to what I had posted earlier about X-bracing.

First, back to an oft-preached basic. The path to structural efficiency - what this event is scored/won on - is a) understanding what each piece in the structure is doing - what kind of force, and how big a force it has to carry, and b) using just enough wood to allow it to do what it needs to do.

The purpose/function of X-bracing - between two ladder pieces that are joining two parallel compression pieces- the only place where it is needed, and where it is important and needed - is to stabilize/fix in space the corners of the rectangle(s) formed by the ladder pieces and compression members they join. In the picture provided, this is the corners of the X. To really understand what I'm about to say, take some of your wood and put together something to feel. 2 pieces, oh, maybe 3" long, joined by two ladder pieces, oh, maybe 2 1/2" apart. Gently move the long pieces back and forth (moving them parallel to each other). The rectangle you start with will shift to a parallelogram; the corners will move. If this sort ov movement were to ocurr in a bridge, the bridge will distort, and fail. Now, take two piece of sewing thread. Run (as in glue) them attaching opposite corners - as in make an X. Pull them taught as you glue them. Now try to deform that rectangle into a parallelogram. You'll find that, essentially, you can't. The X-members, acting in tension (ONLY) prevent that distortion. All you need to stabilize the rectangle is X-pieces acting in tension. To do that, they don't need to be .....sticks- they just need to be strips (which weigh a lot less). The X as shown uses a lot more wood (as in adds weight) than necessary.

[StampingKid]

Balsa Man is there a reason for x bracing over z bracing which would use 1/2 the wood? If the forces are not great it would seem that you might get away with z over x.

[StampingKid]

And should medium density 1/64 be sufficient?

[Balsa Man]

Balsa Man is there a reason for x bracing over z bracing which would use 1/2 the wood? If the forces are not great it would seem that you might get away with z over x.

Z-bracing- a single strip joining only two corners of the rectangle only prevents movement/distortion in one way; X-bracing prevents it in both ways. For the structure to be stable, you have to control both possibilities

And should medium density 1/64 be sufficient?

Yes.... We use 1/16th wide (or a hair wider), typically from a 3"x36" sheet weighing between 8 and 9 grams, which in terms of "medium"/"heavy" as used by Specialized Balsa really falls into heavy. Testing indicates "medium" should be strong enough, but the difference is so small we go with the safety factor "heavy" provides. If "medium" is what you have, maybe you cut a tad wider....

[Aia]

I uploaded my remaining bridges to the image gallery. Unfortunately, I couldn't find photos of one of my final bridges... nonetheless, 41 of my 42 bridges are now up for viewing. Enjoy!

[SOCoach]

Aia,

Let me say your pictures have been very helpful to my bridge builders - using some of your ideas they finally built a bridge that held all the weight (it was their 17th bridge this season). It weighed just under 13 grams and scored right around 1200 points. I hope they took enough detailed notes to replicate the process for our state tournament!

One question on your later bridges (40's). Did you find the major compression pieces were easier to make by doubling them and sandwiching them together between other pieces . . .and if I can ask, what sized pieces did you use?