Elevated Bridge B/C

[Aia]

Yeah Aia, I'll be building this year for sure. This is my last year though.

Did you get a chance to build bridges in B division?

In addition, what'd your boom look like at Nationals? Did you ultimately compete with a box beam? I'm hoping that since the 2008 season is over now, we don't have to be quite as secretive with boom designs and such.

[dudeincolorado]

okso im starting to build monday and i want to do well should i start with over doing it or under doing it? like should i bring it up to 15 kg or should i bring it down to 15kg

[Aia]

okso im starting to build monday and i want to do well should i start with over doing it or under doing it? like should i bring it up to 15 kg or should i bring it down to 15kg

I would attempt to overbuild first, then bring down the mass of your structure. Actually, in this point in the game, I would attempt several designs and see which ones give the best results. These would be my two points for the first few structures... I think you'll find it difficult to hold a bunch of weight on the first try, or at least I did.

Hey colorado, you should let us know how it turns out. I know I'm curious to see what others are trying.

[Pleiades]

I'm hoping to get some balsa by the end of september. I don't have any hobby shops around here so i guess I'm stuck buying balsa online unless Guillow's sells balsa which they might but i doubt they'll have specific grains and densities. I'm assuming that most of the balsa should be 1/8 C grain 5-6lbs/ft2 with some A/B grains right? I'm not really sure since I'm new to this. Once i build some bridges i'll let you guys know how they come out.

[dudeincolorado]

ok thanks i was going to try diffrent desgins (right now i have two, one with all the beams leading to a center point, and one that just looks like a normal bridge) grr i need to find somthing that mesures mass i try to post results asap

[captbilly]

I've thought about arching balsa... but it seems very difficult. To get a perfect arch, the balsa would have to be the same density throughout the stick, then multiply that by two for each side. When a friend of mine was competing in gliders, she told me that she baked the balsa somehow to get the arches.

What advantage would an arch give anyways? Would you be gaining strength or losing mass by using them?

Besides the A frame, I've also built a bridge that looks similar to the example on the rulesheet. However, I changed up the crossbeams and main supports to what I thought would be more efficient.

I have seen some arched bridges and heard about many attempts to build them, but I can think of no engineering reason why you would want to. An arch can be very effective in supporting distributed loads but it is actually less efficient than straight structural members when supporting point loads. Since SO bridge is a point load there would be little reason to make an arch. The kid who built the most efficient bridges at Nationals in 05 and 06 always used straight structures and he beat everyone by at least 25 % both years. Perhaps will will be building in C division this year since he was in 6th grade in 05, we'll see.

[dudeincolorado]

arching hmmm well if you can do it that would be alot less work that and that its really pretty but i agree doesnt seem to help

[andrewwski]

I've thought about arching balsa... but it seems very difficult. To get a perfect arch, the balsa would have to be the same density throughout the stick, then multiply that by two for each side. When a friend of mine was competing in gliders, she told me that she baked the balsa somehow to get the arches.

What advantage would an arch give anyways? Would you be gaining strength or losing mass by using them?

Besides the A frame, I've also built a bridge that looks similar to the example on the rulesheet. However, I changed up the crossbeams and main supports to what I thought would be more efficient.

I have seen some arched bridges and heard about many attempts to build them, but I can think of no engineering reason why you would want to. An arch can be very effective in supporting distributed loads but it is actually less efficient than straight structural members when supporting point loads. Since SO bridge is a point load there would be little reason to make an arch. The kid who built the most efficient bridges at Nationals in 05 and 06 always used straight structures and he beat everyone by at least 25 % both years. Perhaps will will be building in C division this year since he was in 6th grade in 05, we'll see.

Ah, very true. It's a whole different story when talking about point loads (in model bridges I've built in the past I use K-trusses over any other type).

[rjm]

Arches work best when the bearing points have solid foundations, or a direct tensile tie from end to end. The bridges this year are not allowed a direct tie between bearing points, and also can't gain any resistance to spreading from the testing surface. Bridges must be rigid enough to resist spreading at their bases. Whatever advantages an arch may have had, will be largely voided by the clearance requirement. That's really the difference between elevated bridges and the old bridges: designing stability into them will be more complex.

Hopefully there will be some very creative designs, since this is a new variation on bridges. A good starting point would be to build a bridge similar to '05 and '06 bridges and build a frame or legs on the ends. This may not end up being the most efficient approach, but it will get you started.

The problem with putting an illustration in the rules is that folks may think that's what a bridge should look like. It is actually just there to illustrate the rules; it is not intended to be a design example.

Bob Monetza
Grand Haven, MI

[dudeincolorado]

mmm anyone know a good place for balsa thats actually a store like walmart ?