Elevated Bridge B/C

[AlphaTauri]

Soak it in water?

We tried to make an arch with balsa wood last year by soaking it in water then bending it and gluing. It didn't work very well- we snapped every piece of balsa we tried to bend. But that may have been because we weren't very patient and didn't let the wood soak for long enough. Definitely not trying that again this year.

[2win]

okay, thanks, i'll try that.

[nejanimb]

I would just recommend not doing it. Arches, from everything I've ever seen, don't work too well.

[2win]

really? okay, i'll talk to my partner about it, thanks.

[AlphaTauri]

Yeah, arches don't work very well because you can't connect the ends to keep the wood in its curved shape. Since the wood wants to spring back to it's original shape of being straight, the legs are going to want to slide outwards and the middle of the bridge is probably going to bend and break. Basically anything curved is a bad idea in a SO bridge.

[andrewwski]

Arches aren't really worth it, from my experience.

You don't want to just bend a piece of wood - now you have forces created within the bridge itself - that's bad. If you can get it bent into a permanent shape without breaking the grains, it will work, but it's not worth the time and effort.

[Greg Doe]

Just when you think you have figured something out, along comes someone who thinks they're an expert! Our situation is unique in that I coached Elevated Bridge (B Div.) at two schools last year. My students finished first and second at both regionals and state with "Arch" bridges. By the way neither
I, nor my students are experts, and our design probably wouldn't have medaled at nationals, but it was good enough to win at the state level. The bridges were built with 6 to 10 lb. per cu. ft. balsa
wood. The wood was soaked in hot water, glued with wood glue, and dried next to a heater. The kids broke a few sticks, but we always cut extras and pitched the broken ones. So it can be done, and possibly with more refinement an arch bridge could place or win at the SO Nationals. Good Luck.
Greg Doe
Smyrna, TN

[StampingKid]

Built first B bridge this weekend. 10 grams--layoff from last season was costly in execution with way too much glue. Failed at 10 kg on a very weak tension chord. But regardless it is going to be hard to improve on last year's efficiencies. The extra 5 centimeters of height added a minimum of 40 centimeters of wood on the legs and tension arms. The 15 centimeter clearance block added an extra couple of centimeters on the top chord and the chord sagged under the loading block. The tension chord that failed ran from the base to the top chord and I guess the sag was indicative of the forces. The neat thing was the tension chords were so tight you could almost play them like a guitar string. But alas the dream of building one bridge and moving on to Wright Stuff flying is over for another year.

[blue cobra]

My partner only builds arches. At States, the sides were perfectly intact, but the cross members were what broke. We still did well, however. So, arches can be done well, and you can build arches without too much difficulty.

Another design that seems to do well, and beat us at regionals, is an arch made of straight pieces. If you look at one of my last posts, I posted a picture of my bridge that incorporates that (I couldn't do it for that bridge, but it CAN be done well).

The added height is proving to be quite a challenge

[andrewwski]

My partner only builds arches. At States, the sides were perfectly intact, but the cross members were what broke. We still did well, however. So, arches can be done well, and you can build arches without messing around with soaking the wood and all that.

How is he getting the arch without soaking the wood?

I suppose if you could find a sheet of balsa that's large enough, you could cut it out of that. But I don't know where you'd find that large of a sheet.

Another design that seems to do well, and beat us at regionals, is an arch made of straight pieces. If you look at one of my last posts, I posted a picture of my bridge that incorporates that (I couldn't do it for that bridge, but it CAN be done well).

The added height is proving to be quite a challenge

Yes, but at which point does it become an arch? The rounder you make it, the more joints you have to use. The more joints you use, the heavier the bridge gets.

[2win]

Ohhh, okay, thanks guys!
(for the second idea on andrewwski's post) I suppose it doesn't have to be that much of an arch, just similar? like, not a lot of joints to make it heavy? ... i don't know, i'm not good at this events.

[smartkid222]

theoreticly an arch may be the best design, but when putting it into practicle use, its not worth it.
If you use a lot of joints, you need to use glue on each of those joints, making them heavy. it also increase the risk of failure due to a bad joint.

[sewforlife]

Can you get graph paper big enough to fit an entire bridge on it? Right now I tape two pieces together, but the tape can stretch and slip.

i kno on paperprint.com you can print out cm graphs which is better, because the lines def help me more than the graph paper in inches. i just tape together and make sure they're exact

[sewforlife]

hmm. this year's bridge for div. B is a lot dif. I was going to use a bridge with about 1000 efficiency from last year, but the angles are off. I guess I'll just test it as an experiment. I hope it goes well. I think I'll try to post some pics up soon.

[fmtiger124]

I have to build one side of an Bridge for my SO tryouts. I was wondering what you guys thought in regards to how light/heavy and simple/complex it should be. I would think starting heavy and then trying to go lighter would work best when building a series of bridges however, I'm not sure how that would work to start heavy from a tryout perspective.