Bridge Weight - Division B

[Unome]

I was wondering about this. I am not sure how to ask this, but is it referring to the center line on the x or y axis? I am not sure how else to phrase this. Center line of the width OR length of the bridge? Maybe that is better.

I see two rules, 3.e. and 5.b.iv.. When considered together, the rules seem specify a location for the loading block both along the Bridge span and along the test base. 3.e., the rule that specifies a location along the Bridge span has it at the center (along the length). 5.b.iv., the rule that specifies a location relative to the test base could be interpreted as precisely in the center as possible - centered both in the directions of the width and length of the bridge. And it seems possible for a setup to follow one rule but not the other.

I can definitely see how event supervisors could struggle interpreting/implementing the wording of those specific rules. Fantastic diagrams- thank you Bernard!

That's a tough one... I would have tiered the last one, but not the other two, since the rules mention "aligned with the centerline" and the centerline is indicated on the diagram in the rules.

[bernard]

I see two rules, 3.e. and 5.b.iv.. When considered together, the rules seem specify a location for the loading block both along the Bridge span and along the test base. 3.e., the rule that specifies a location along the Bridge span has it at the center (along the length). 5.b.iv., the rule that specifies a location relative to the test base could be interpreted as precisely in the center as possible - centered both in the directions of the width and length of the bridge. And it seems possible for a setup to follow one rule but not the other.

I can definitely see how event supervisors could struggle interpreting/implementing the wording of those specific rules. Fantastic diagrams- thank you Bernard!

That's a tough one... I would have tiered the last one, but not the other two, since the rules mention "aligned with the centerline" and the centerline is indicated on the diagram in the rules.

I thought that initially, but reading the rules again I realized that it doesn't mention the centerline. It species a location relative to the Bridge span (3.e.) and relative to the Test Base opening (5.b.iv.).

[JonB]

I can definitely see how event supervisors could struggle interpreting/implementing the wording of those specific rules. Fantastic diagrams- thank you Bernard!

That's a tough one... I would have tiered the last one, but not the other two, since the rules mention "aligned with the centerline" and the centerline is indicated on the diagram in the rules.

I thought that initially, but reading the rules again I realized that it doesn't mention the centerline. It species a location relative to the Bridge span (3.e.) and relative to the Test Base opening (5.b.iv.).

Do we believe that this interpretation of the rules is what was intended from the rule writers (that it must be BOTH relative to the span and test base opening)?

[SPP SciO]

Rule 5.b.iv includes the word "approximately." I think that alone is enough to accept any of the 3 bridges represented by the diagrams. Plus, when you take into account General Rules #1 and #5, I think the only sane conclusion is, kids should not be tiered for slightly off-center loading blocks. What unfair advantage would they be gaining? The "spirit of the rules" seems to be, "all bridges will be loaded from the middle." If I were an ES, I'd interpret "approximately" to mean, as long as the 25 square centimeters of the block covers the exact center point of the opening, it's good to go.

Also, the checklist on soinc.org notes 3.e and 5.b.iv as two separate criteria. If we wanted to get technical with semantics, couldn't a student show a judge during check-in that, yes, a loading block will indeed fit exactly in the center of the bridge - getting a Yes check for 3.e - but then during competition, actually placing the loading block approximately in the center, wherever it best works for their design?

[dholdgreve]

Rule 3 applies to "construction Parameters... The bridge must be built to accommodate a loading block placed in the center... This rule does not require that the loading block be placed there, only that the bridge be built to accommodate one at that location. Rule 5 refers to the actual testing, and is more lenient, allowing the loading block to be placed "approximately" in the center. Although this could get subjective, I believe most E/Cs will allow a pretty wide variation from dead center... Personally, I believe that the center of the loading block needs to be within 2.5 CM either side of the actual center of the bridge to even remotely qualify as "Approximately." beyond that, the loading block would not even touch the actual center, which I personally feel is no longer "approximately"... but that is just my personal feelings... I can't image tiering someone for not meeting this definition.

[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.

[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.

[Mr_Pep_Band]

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.

[dholdgreve]

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.