Designs B/C

dholdgreve
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Re: Designs B/C

Post by dholdgreve » March 26th, 2015, 8:09 am

I think the choice to pretest or not depends entirely on the situation. Do you have a fall back bridge? One that you can use and get a respectable score, in case this one fails? Do you have enough time to build a new one if this one fails? Always leave yourself a plan B. Personally, I do not believe that pretesting weakens the structure as much as the age of the glue does. I seen it happen many times where a bridge is tested and carries the full load at 2, maybe 3 invitationals over a couple month period, then it gets tested again, and fails at 12 KG, and everyone thinks that it is because it has been tested just once too often and never considers that the cyanacrylate glue is now 4 months old, has lost all flexibility and become very brittle. This would have happened whether the bridge was tested multiple times or not. Two opposing schools of thought I guess.

We try to develop a bridge that weighs as little as possible that carries the full load, then build another, lighter bridge and test it only to the score of the first bridge. This is always something less than the 15 Kg, since it is lighter than the first bridge. If it carries that load, we can be assured that it will outperform the first bridge, since there is still additional load that can be added to the second bridge.
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Re: Designs B/C

Post by SO_2015 » March 26th, 2015, 10:12 am

Thank you.

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Re: Designs B/C

Post by sjwon3789 » April 12th, 2015, 9:34 am

dholdgreve wrote:I think the choice to pretest or not depends entirely on the situation. Do you have a fall back bridge? One that you can use and get a respectable score, in case this one fails? Do you have enough time to build a new one if this one fails? Always leave yourself a plan B. Personally, I do not believe that pretesting weakens the structure as much as the age of the glue does. I seen it happen many times where a bridge is tested and carries the full load at 2, maybe 3 invitationals over a couple month period, then it gets tested again, and fails at 12 KG, and everyone thinks that it is because it has been tested just once too often and never considers that the cyanacrylate glue is now 4 months old, has lost all flexibility and become very brittle. This would have happened whether the bridge was tested multiple times or not. Two opposing schools of thought I guess.

We try to develop a bridge that weighs as little as possible that carries the full load, then build another, lighter bridge and test it only to the score of the first bridge. This is always something less than the 15 Kg, since it is lighter than the first bridge. If it carries that load, we can be assured that it will outperform the first bridge, since there is still additional load that can be added to the second bridge.
Do you think it's the same case for all bridges if you don't get a new glue soon? I've been using the same glue probably for more than 4 months (I'm referring to the exact same glue, not the same brand or anything lol).


Also, looking at elevated bridges on the Best of 2010 and 2009, I noticed that those students (neja something) built something in the center. I believe they curved out the main compression members in the center and attached supporting members in there. Am I correct? What would you do to curve it out though, because I don't think it'll be consistent over time?

Do you guys use a scale to the nearest hundredth or thousandth? Do you guys measure out each individual compression support members? I think it would be insignificant to measure those since they wouldn't vary so much. I would always just weigh the 36" sticks and long pieces that I cut out. That should be enough right?
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Re: Designs B/C

Post by dholdgreve » April 13th, 2015, 10:39 am

It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.

We use a scale that goes to the hundredths... I think going to the thousandths of a may require a dome, or some sort of protection to be accurate. I try to get the builders to document every single piece that goes into a bridge... Just quickly weigh it, then write it down on their template, next to the piece. At our Regionals, they only weighed the bridges to the nearest tenth of a gram, which IMHO is not accurate enough. When bridges were "in" several years ago, several teams tried an arch design, but I do not believe the arch has near the potential efficiency as the tower type bridge that is so popular this year. In 2005, I believe the winning mass in Div B Ohio was about 6.23 grams... This year I'm sure it was under 4 grams... This just goes to show how much more competitive Science Olympiad has become.
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Re: Designs B/C

Post by sjwon3789 » April 19th, 2015, 10:15 pm

dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.

We use a scale that goes to the hundredths... I think going to the thousandths of a may require a dome, or some sort of protection to be accurate. I try to get the builders to document every single piece that goes into a bridge... Just quickly weigh it, then write it down on their template, next to the piece. At our Regionals, they only weighed the bridges to the nearest tenth of a gram, which IMHO is not accurate enough. When bridges were "in" several years ago, several teams tried an arch design, but I do not believe the arch has near the potential efficiency as the tower type bridge that is so popular this year. In 2005, I believe the winning mass in Div B Ohio was about 6.23 grams... This year I'm sure it was under 4 grams... This just goes to show how much more competitive Science Olympiad has become.
Actually, I did understand you correctly - I was just extending on your point and asking a question from there. Just curious whether my glue had an effect on it because sometimes the supporting compression members fell out after testing. I guess it wasn't dense enough?

I always wrote the weight of each pieces on the wood itself... Wouldn't it be less organized to write it next to the template, since you will need to keep the template themselves? When I go back to the bridges I tested, I can always see the weight of the bridges and make easy note of each variables. I would also always use the same template over and over until I have to change my design.

I always see every teams attempting the arch designs because it is efficient to some extent -- until they discover a more efficient design, although I have not attempted them because I hear how inconsistent they are for each testing. I believe the score in division C is around 1600-1800 for arch bridges, whereas the tower-like design has the potential to go above 2000.
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Re: Designs B/C

Post by DoctaDave » April 19th, 2015, 10:22 pm

sjwon3789 wrote:I always see every teams attempting the arch designs because it is efficient to some extent -- until they discover a more efficient design, although I have not attempted them because I hear how inconsistent they are for each testing. I believe the score in division C is around 1600-1800 for arch bridges, whereas the tower-like design has the potential to go above 2000.
Not quite true.

I've seen several teams getting well over 2500 with an arch in C div. They seem to have the advantage of stability, since the loading block usually sits on the bottom, so it's less likely to tip over than the "tower" design, although I do believe the tower design loaded from the top has more potential than an arch loaded from the bottom if built correctly. But more importantly, arch bridges are just absolutely gorgeous :D

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Re: Designs B/C

Post by DoctaDave » April 19th, 2015, 10:42 pm

dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.
Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.

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Re: Designs B/C

Post by bernard » April 19th, 2015, 11:21 pm

DoctaDave wrote:
dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.
Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.
Would be interesting to do some tests and leave some joints for a while. I might do that if I ever get bored. I've had several bottles of CA glue this season and I've left some of them open for a while. Some have hardened and some are still as viscous as when I bought them. I'm not sure why, but just what I've noticed.
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Re: Designs B/C

Post by dholdgreve » April 20th, 2015, 9:01 am

bernard wrote:
DoctaDave wrote:
dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.
Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.
Would be interesting to do some tests and leave some joints for a while. I might do that if I ever get bored. I've had several bottles of CA glue this season and I've left some of them open for a while. Some have hardened and some are still as viscous as when I bought them. I'm not sure why, but just what I've noticed.
I have no empirical data to support that claim that C/A glue will get brittle over time, but I have seen it fail multiple times after being set on a shelf for a few months, after having held without any issues 2 or 3 times when first built. This, with no additional sanding or any other modifications, and yes, the joint actually split apart with both piece of wood intact... totally glue failure.
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Re: Designs B/C

Post by AnonymousMouseRI » April 25th, 2015, 6:55 pm

How has the pratt truss worked out? We tried it at states (even though we got it together in about a day), and it only held 17 lbs. We have to finagle a new design for nationals. Any ideas? I am sort of at a loss.

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