Bridge Weight - Division B

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Re: Bridge Weight - Division B

Post by SPP SciO »

With regards to bridge weight and the original problem in this thread - is it more prudent to remove members outright, or just switch to lighter members? I'm curious if these super light, high scoring bridges are just a few well placed members, or complicated and intricate structures with tiny members?

The first bridges my students built were relatively heavy, ~20 grams. Instinctively I find that kids choose heavier than necessary wood; it's easier to work with and there's some skepticism that members so tiny (1/8" and below) could possibly be strong enough. So, for the next batch, I had them select wood before designing and work with more constraints. The lightest they've achieved has been ~5g, but they visually look imperfect and don't score very high. There were a lot more glued fingers and tricky moments with the 1/16" balsa they tried. I think there is a "ceiling" to how high kids can score based on their manual skills.
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Re: Bridge Weight - Division B

Post by XplodingCesium »

You want your bridge to be as light as possible, yet hold as much weight as possible.
The lighter the bridge is, the more opportunity that there is to have a higher efficiency.
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Re: Bridge Weight - Division B

Post by Mr_Pep_Band »

SPP SciO wrote:With regards to bridge weight and the original problem in this thread - is it more prudent to remove members outright, or just switch to lighter members? I'm curious if these super light, high scoring bridges are just a few well placed members, or complicated and intricate structures with tiny members?

The first bridges my students built were relatively heavy, ~20 grams. Instinctively I find that kids choose heavier than necessary wood; it's easier to work with and there's some skepticism that members so tiny (1/8" and below) could possibly be strong enough. So, for the next batch, I had them select wood before designing and work with more constraints. The lightest they've achieved has been ~5g, but they visually look imperfect and don't score very high. There were a lot more glued fingers and tricky moments with the 1/16" balsa they tried. I think there is a "ceiling" to how high kids can score based on their manual skills.

I would say use lighter members. In most structures there are "zero force" members that can be minimized.
However, this in in theory. In reality there is a bend in a member, a joint is not glued perfectly, etc.
Meaning there are imperfections in construction, its unavoidable.

Larger members are easier to work with yes, but, they are not as efficient as smaller members.
I believe this is due to the ratio of member volume vs member surface area per unit length of said member.

From my observation and experience, structures that use smaller members for this competition generally perform better.
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Re: Bridge Weight - Division B

Post by dholdgreve »

SPP SciO wrote:With regards to bridge weight and the original problem in this thread - is it more prudent to remove members outright, or just switch to lighter members? I'm curious if these super light, high scoring bridges are just a few well placed members, or complicated and intricate structures with tiny members?

The first bridges my students built were relatively heavy, ~20 grams. Instinctively I find that kids choose heavier than necessary wood; it's easier to work with and there's some skepticism that members so tiny (1/8" and below) could possibly be strong enough. So, for the next batch, I had them select wood before designing and work with more constraints. The lightest they've achieved has been ~5g, but they visually look imperfect and don't score very high. There were a lot more glued fingers and tricky moments with the 1/16" balsa they tried. I think there is a "ceiling" to how high kids can score based on their manual skills.
Keep in mind that the more parts there are to a bridge, the more joints there are... The more joints there are, the more chance that one of them will fail. A bridge is just like a chain... It is only as strong as the weakest link. Think simple, build light... test while video taping. Slow it down and see which member failed first, then increase either the density or size (or both) of just that member, and build another... then repeat... over and over... The last few competitions I have been to, even to land a bronze, it has taken a score of over 3300... That's a bridge carrying the full 15,000 grams, weighing in at a hefty 4.54 grams... basically the weight of a nickel.
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