Elevated Bridge B/C

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Re: Elevated Bridge B/C

Post by Balsa Man » February 11th, 2010, 3:30 am

Flavorflav wrote:How are B and C efficiencies comparing this year?
I'm working with both our High School, and a local Jr. High this year; very similar designs, just dimensioned differently. No full structure test data yet, but the B-Div bridge looks like its going to come in about a gram lighter than the C-Div. That's a 10%, or a little more difference. How that difference looks with other designs; don't know. The trade-off, like last year is B-Div doesn't have to cover as much span (= less lumber), but the forces in the truss, due to lower truss height, are greater (= heavier lumber and/or more column bracing)
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Re: Elevated Bridge B/C

Post by blue cobra » February 11th, 2010, 2:12 pm

xXRichcash1Xx wrote:Can you bend balsa? Or does basswood bend better? I don't know which one. I read a chart that said that balsa was stronger pound for pound. Is that true?
I would assume since bass is stiffer than balsa it would be much harder to bend. Either way soak your wood overnight, then make a jig to form your arch (eg a board with nails, a piece of hardwood cut to your shape, or maybe very careful pinning). It's probably best to use layers: an 1/8 square piece might break, but if you glue together four 1/8x1/32 pieces after bending they probably won't break (sorry if this is confusing). But I would suggest you don't make an arch anyway.

Typically, I think that balsa is a stronger for its weight than bass, but remember that if it breaks too early, that doesn't matter. And you have to find a piece of balsa the right density and stiffness.
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Re: Elevated Bridge B/C

Post by baker » February 11th, 2010, 4:01 pm

2win wrote:i think balsa's better, base on personal experience. and i've heard people try putting it over fire, and baking it? idk...
It's still too chewy for me, too much fiber :D

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Re: Elevated Bridge B/C

Post by xXRichcash1Xx » February 11th, 2010, 5:10 pm

Huh. I found that Balsa is stiffer than Basswood. O and I just put my wood over the stove and bend it by hand. Then i but it on a chart to check if it is the correct shape. I have made a pretty arch. I don't really wanna do the hot water because it seems really complicated. This is my first time building abridge so I guess i must be kinda "Stupid" here. Why not do a arch? I thought it would be very strong. All you have to do is prevent it for expanding. I was going to make a typical bridge but i was worried that all the weight would be put on the joints.
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Re: Elevated Bridge B/C

Post by cypressfalls Robert » February 11th, 2010, 6:16 pm

a good way to keep it from going back to its original shape is to bake the arc right after it drys.

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Re: Elevated Bridge B/C

Post by Jest » February 11th, 2010, 8:03 pm

In my experience, making arches have been too much of a hassle to make with steaming or boiling it in water and then pinning it down with nails and such. Even when I was able to make a satisfying arch, they seem to be inconsistent for what ever reason it might be.

But, if you get it to work, more power to you.

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Re: Elevated Bridge B/C

Post by jazzy009 » February 11th, 2010, 8:14 pm

xXRichcash1Xx wrote:Huh. I found that Balsa is stiffer than Basswood. O and I just put my wood over the stove and bend it by hand. Then i but it on a chart to check if it is the correct shape. I have made a pretty arch. I don't really wanna do the hot water because it seems really complicated. This is my first time building abridge so I guess i must be kinda "Stupid" here. Why not do a arch? I thought it would be very strong. All you have to do is prevent it for expanding. I was going to make a typical bridge but i was worried that all the weight would be put on the joints.
Arches were once described to me as "the beautiful bridges that get next to last place"
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Re: Elevated Bridge B/C

Post by matthew8817 » February 12th, 2010, 11:13 am

jazzy009 wrote:
xXRichcash1Xx wrote:Huh. I found that Balsa is stiffer than Basswood. O and I just put my wood over the stove and bend it by hand. Then i but it on a chart to check if it is the correct shape. I have made a pretty arch. I don't really wanna do the hot water because it seems really complicated. This is my first time building abridge so I guess i must be kinda "Stupid" here. Why not do a arch? I thought it would be very strong. All you have to do is prevent it for expanding. I was going to make a typical bridge but i was worried that all the weight would be put on the joints.
Arches were once described to me as "the beautiful bridges that get next to last place"
Just saying...
I agree. My partner last year tried all sorts of arch bridges, and although they looked pretty, they didnt do half as well as my simple truss bridges. :|
Just sayin...
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Elevated Bridge = #1
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Re: Elevated Bridge B/C

Post by blue cobra » February 12th, 2010, 12:31 pm

Arches are great in theory but never seem to work in practice for this event. Basically, it's a very long piece of highly variable wood, and you're introducing forces within the bridge. You can look through this and last year's thread for more.

My partner always builds arches, and they do decently, medaling at Invites and Regionals, but my simplistic bridge just got nearly three times its efficiency. Also, taking the example of his bridge, if you moved a part of the arch, a bit farther up it would move pretty far, and in a bizarre fashion. Hard to describe, but those sorts of things tend to pop up in arches.

But by all means, experiment with everything! Arches are still great in theory, and if you can pull it off, don't let us stop you. This is Science Olympiad not a trust-strangers-on-the-internet competition. I'd just recommend that you also experiment with simpler designs. And as for your worries about all the weight being on the joints, with most glues the joint is stronger than the members the are connecting (IF they are built well!), so I wouldn't worry too much about it, just make sure your members are flush before gluing.

Now, I have a question: When I sand my pieces flush, the middle sometimes sticks out a little; maybe 1/64 or 1/32. Is there any technique you can use to stop this, besides from being a robot that can sand perfectly (which we all aspire to be).
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Re: Elevated Bridge B/C

Post by WrightStuffMonster » February 12th, 2010, 1:10 pm

blue cobra wrote:
Now, I have a question: When I sand my pieces flush, the middle sometimes sticks out a little; maybe 1/64 or 1/32. Is there any technique you can use to stop this, besides from being a robot that can sand perfectly (which we all aspire to be).
Why are you sanding in the first place. It decreases strength (because the sand paper tears at the outer cell walls of the balsa) increases density and is not as good as making a straight accurate cut in the first place. What are you doing that requires sanding? If you are just sanding the end of a stick or something the best way to make sure that they are flat is to build a jig so that the stick is always held firmly upright. I can probably draw a picture of one like I used to use if you would like to see it.
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