Elevated Bridge B/C

[Balsa Man]

The major problem with arches is that you have to weaken the wood to get the curve. In addition, curves are hard to get symmetric. Any balsa stick is inherently different from another, so the curves vary a little from one side of the bridge to another.

Good analysis, Aia,

Precision/symmetry is so important to keeping forces at/near design levels. With curved pieces, not only do you have the very difficult challenge of getting a symmetric curve built, the differences in stiffness as you move along a stick will make it flex asymmetrically under load. That will take the bridge out quickly. A straight piece under compression - properly braced- will take quite a load without flexing at all; in a curved piece under load, wherever the softest zone is, it will flex.....quickly. As andrewwski says, great in theory, but not in balsa....

I saw a lot of arch bridges this year, but none of them had any success

Me, too, and some were very......pretty ones. No good efficiencies, though.

[rocketchicka]

I agree with you guys all the way. I made an arch for regionals and got fourth but it took forever to get it right. 28 hours of work for a "pretty" bridge is not worth it. It wouldn't do well at states.

[ktownpa7]

if you can find thhe right way to use an arch, then they work really well.

[Valpo Towers and Trebs]

if you can find thhe right way to use an arch, then they work really well.

What kind of efficiency is "really well"?

[andrewwski]

The issue is not so much how to use the arch as it is how to build the arch.

[fli754]

The issue is not so much how to use the arch as it is how to build the arch.

Yeah, the method I used to make an arch was to reinforce it by taking two 1/8" sq balsa sticks, soaking them in warm water, and then using elmer's glue to connect them together, and then using a template to mold them into an arch shape and have it dry over night. Looking back to it now, it was a really bad way to create and arch lol

[sewforlife]

Not sure if this was said or not, but is an arch design for a bridge better or worse than a uhh.. not arch design?

hmm. I saw a lot of arches as well. the only clever design I saw was a small arch and it had a support going from one end to the other, which was just in case the arch wasn't symmetrical. but it looked pretty symmetrical to me.

[fli754]

Not sure if this was said or not, but is an arch design for a bridge better or worse than a uhh.. not arch design?

hmm. I saw a lot of arches as well. the only clever design I saw was a small arch and it had a support going from one end to the other, which was just in case the arch wasn't symmetrical. but it looked pretty symmetrical to me.

Do you know if it did well or not?

[nejanimb]

The best I've seen an arch do is a bit less than 900. Definitely more trouble than it's worth.

[Greg Doe]

The kids I coached (2 different schools) built lamanated bridges which finished first and second at
state. First was over 1400 efficiency. However I believe that particular design would of maxed out
around 1500 plus or minus a 100 or so. Neither team won overall so no trip to the nationals. We laminated wet 3/32 in.strips of 6 to 10 lb.per cu.ft. balsa with thinned carpenters glue (yellow
alaphetic glue). Multiple lamanations tends to reduce the effects of variations in the wood.
In buildings sometimes you can see structural laminations (referred to as Structural Lams) in the form
of pre laminated curved beams that go up the walls and across the roof. They are often seen in churches and gymnasiums.
By the way bending the wood does not weaken it. Wood does not fatigue or strain harden. So long as
you do not exceed it's bend limits (fracture) it is not compromised.
Instead of building a lot of bridges, we repaired our prototype several times, strengthing the areas that failed. One early bridge was repaired 5 times.
This was all B division. Lamination would probably be less effective in C division.
Greg Doe
Smyrna,TN