Bridge B/C

dholdgreve
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Re: Bridge B/C

Post by dholdgreve »

jgrischow1 wrote: April 11th, 2022, 5:49 am This forum is pretty dead so I thought I would enliven it with some hot silica talk.

So my kid had a bridge that he pre-tested and it held the whole weight. He put it in silica night before State tourney and it dropped a further .18 grams. Tested in on camera for the remote tourney and it broke holding 14,968 grams...32 from the bonus!!! :(

So my question is did we do the right thing. Our options were:

1. No silica and just test it on camera knowing we pre-tested it.
2. Silica and pre-test it again right before filming knowing it will regain some weight and every additional test means closer to breaking
3. Silica and test it on camera...what we did

Obviously it's impossible to know if it broke because drying it made it more brittle or it only had one test in it or whatever...but is there an optimal strategy?
I feel your pain! We had a bridge come up just 6 grams short of a full load at the National Invitational tournament! I do not subscribe to the belief that testing the bridge to full load somehow weakens it. We have used balsa wood structures over and over (as many as 8 times) with much better results than taking an untested or partially tested structure to a competition. I can see how drying it down too much could be problematic, either through the use of silica or baking. I know there are those that will disagree, but my belief is that C/A glue will become brittle as it continues to cure. As such, your risk of it breaking increases after 90 days or so... Just my thoughts.
Dan Holdgreve
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Dedicated to the Memory of Len Joeris
"For the betterment of Science"
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Re: Bridge B/C

Post by dholdgreve »

jgrischow1 wrote: April 11th, 2022, 5:49 am This forum is pretty dead so I thought I would enliven it with some hot silica talk.

So my kid had a bridge that he pre-tested and it held the whole weight. He put it in silica night before State tourney and it dropped a further .18 grams. Tested in on camera for the remote tourney and it broke holding 14,968 grams...32 from the bonus!!! :(

So my question is did we do the right thing. Our options were:

1. No silica and just test it on camera knowing we pre-tested it.
2. Silica and pre-test it again right before filming knowing it will regain some weight and every additional test means closer to breaking
3. Silica and test it on camera...what we did

Obviously it's impossible to know if it broke because drying it made it more brittle or it only had one test in it or whatever...but is there an optimal strategy?
Dan Holdgreve
Northmont Science Olympiad

Dedicated to the Memory of Len Joeris
"For the betterment of Science"
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Re: Bridge B/C

Post by mklinger »

Tall wrote: April 11th, 2022, 8:53 am
jgrischow1 wrote: April 11th, 2022, 5:49 am This forum is pretty dead so I thought I would enliven it with some hot silica talk.

So my kid had a bridge that he pre-tested and it held the whole weight. He put it in silica night before State tourney and it dropped a further .18 grams. Tested in on camera for the remote tourney and it broke holding 14,968 grams...32 from the bonus!!! :(

So my question is did we do the right thing. Our options were:

1. No silica and just test it on camera knowing we pre-tested it.
2. Silica and pre-test it again right before filming knowing it will regain some weight and every additional test means closer to breaking
3. Silica and test it on camera...what we did

Obviously, it's impossible to know if it broke because drying it made it more brittle or it only had one test in it or whatever...but is there an optimal strategy?
This is from my experience. The same thing happened to me as the previously held full weight bridge broke at 14.2 kg when it was inside the Silica box.
I guess we need some moisture at least. Less moisture makes it brittle. Also, pre-testing to full weight weakens the structure.

My experience is that just using silica packs to dry out your structure by ~2-2.5% will not negatively effect the strength. Baking down 5-6% is a different story and is more risky.

I think what you ran into was more of pre-testing too close to its eventual maximum load. It's been my experience that you can only safely pre-test to 75-80% of the eventual maximum load. If you get closer to 90-95%, you risk doing internal damage. One way to try and predict this is to listen very carefully during the loading to see if you hear any "creaking or cracking". That is usually a bad sign.

FWIW, we would never pre-test just once for that exact reason. To really be safe, it's best to pre-test in steps up to a maximum and do it multiple times. Over 6 years and dozens of competitions we never had a device break at less weight than our pre-test.

The problem/challenge of course, is that with the 15kg bonus, to actually guarantee you can hold 15, you need to design to more like 16.5 or 17kg. By the very definition, that is overdesigned. So, every team has to make the choice of risking early failure or guaranteeing a better score, but maybe not a winning score. When there is such a big bonus for holding 15kg, I like to think of it as something like this: 50% chance of 1st or 2nd place with a 50% chance of 10th+ place (for early failure). OR a 90% chance of 3-5th place by properly pre-testing to 15kg with a slightly overdesigned device.

That is also why sometimes you see might higher scores in Invitationals during the year than in important competitions because you can hurt the team a lot more if you go for 1st place and come in 10th.

As for CA getting "old", we used an 18 month old boomi that placed 3rd in Div C Nationals in 2021 and I have re-tested about 10 devices that were all over a year old and they all held more than they originally tested, so I don't subscribe to that theory.

Marc
Last edited by mklinger on April 13th, 2022, 4:54 am, edited 1 time in total.
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Re: Bridge B/C

Post by mklinger »

mklinger wrote: April 13th, 2022, 4:54 am
Tall wrote: April 11th, 2022, 8:53 am
jgrischow1 wrote: April 11th, 2022, 5:49 am This forum is pretty dead so I thought I would enliven it with some hot silica talk.

So my kid had a bridge that he pre-tested and it held the whole weight. He put it in silica night before State tourney and it dropped a further .18 grams. Tested in on camera for the remote tourney and it broke holding 14,968 grams...32 from the bonus!!! :(

So my question is did we do the right thing. Our options were:

1. No silica and just test it on camera knowing we pre-tested it.
2. Silica and pre-test it again right before filming knowing it will regain some weight and every additional test means closer to breaking
3. Silica and test it on camera...what we did

Obviously, it's impossible to know if it broke because drying it made it more brittle or it only had one test in it or whatever...but is there an optimal strategy?
This is from my experience. The same thing happened to me as the previously held full weight bridge broke at 14.2 kg when it was inside the Silica box.
I guess we need some moisture at least. Less moisture makes it brittle. Also, pre-testing to full weight weakens the structure.

My experience is that just using silica packs to dry out your structure by ~2-2.5% will not negatively effect the strength. Baking down 5-6% is a different story and is more risky.

I think what you ran into was more of pre-testing too close to its eventual maximum load. It's been my experience that you can only safely pre-test to 75-80% of the eventual maximum load. If you get closer to 90-95%, you risk doing internal damage. One way to try and predict this is to listen very carefully during the loading to see if you hear any "creaking or cracking". That is usually a bad sign.

FWIW, we would never pre-test just once for that exact reason. To really be safe, it's best to pre-test in steps up to a maximum and do it multiple times. Over 6 years and dozens of competitions we never had a device break at less weight than our pre-test.

The problem/challenge of course, is that with the 15kg bonus, to actually guarantee you can hold 15, you need to design to more like 16.5 or 17kg. By the very definition, that is overdesigned. So, every team has to make the choice of risking early failure or guaranteeing a better score, but maybe not a winning score. When there is such a big bonus for holding 15kg, I like to think of it as something like this: 50% chance of 1st or 2nd place with a 50% chance of 10th+ place (for early failure). OR a 90% chance of 3-5th place by properly pre-testing to 15kg with a slightly overdesigned device.

That is also why sometimes you see might higher scores in Invitationals during the year than in important competitions because you can hurt the team a lot more if you go for 1st place and come in 10th.

As for CA getting "old", we used an 18 month old boomi that placed 3rd in Div C Nationals in 2021 and I have re-tested about 10 devices that were all over a year old and they all held more than they originally tested, so I don't subscribe to that theory.

Marc
Just another quick follow-up. This is how I would approach trying to win with a device that was optimally designed for 15kg.

1. Have a known design and build that has been tested to failure and it's exactly how you want. Say it breaks at 15.3kg. Ideally, the team can reproduce this and has done so multiple times in practice.

2. Rebuild multiple devices for a new competition

3. Put them in a dry-box and then pre-test them all to 12kg 3 times in row. If more than one survives, take the lightest one (they should all be within 0.1-0.2g of each other).

4. Cross your fingers that it will hold 15kg during competition. This is basically the best you can safely do.

The idea is that you are using your previous builds and experience to give you confidence that your device will hold 15kg again like it's done in practice. The team needs to build A LOT of devices to gain this kind of experience. Think on the order of 10-20 devices per student over the course of the season.


If you want to be more conservative, do the same steps above with a build designed for 18kg and pre-test something like 2x to 12kg and 2x to 15kg or design to ~17kg and 2x to 12kg and 1x to 15kg and small fingers crossed (this is what we'd typically do).

FWIW, that is why the 5kg bonus for holding 15kg makes the event MUCH more challenging. They usually wait until the second year of a device to add that scoring tweak and I'm surprised they had it this year instead of waiting for next year.

Marc
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Re: Bridge B/C

Post by JonB »

We have done silica and not done silica... to be honest, it is probably not worth your time, but it might depend on how quickly you can get your balsa structure from the "holding container" (with silica) to the loading process. If there are a few minutes of time between, it will simply re-gain the mass by absorbing moisture from the air (especially here in Florida since we have high humidity). I sometimes feel that silica/drying adds a variable that the students cannot actually know.

Also, as far as CA and weakening- I might argue that not all CA is created equally- some might become more brittle over time, others might not. This is purely conjecture- I have no evidence for this statement but so many of them have other ingredients in them in addition to the CA.
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Re: Bridge B/C

Post by mklinger »

JonB wrote: April 13th, 2022, 5:24 am We have done silica and not done silica... to be honest, it is probably not worth your time, but it might depend on how quickly you can get your balsa structure from the "holding container" (with silica) to the loading process. If there are a few minutes of time between, it will simply re-gain the mass by absorbing moisture from the air (especially here in Florida since we have high humidity). I sometimes feel that silica/drying adds a variable that the students cannot actually know.

Also, as far as CA and weakening- I might argue that not all CA is created equally- some might become more brittle over time, others might not. This is purely conjecture- I have no evidence for this statement but so many of them have other ingredients in them in addition to the CA.
I agree that you need to be able to get the device out and weigh it as quickly as possible. Most competitions we went to, the students would bring the sealed box up to the table and only open it when it was time to weigh the device. It would typically only need to be exposed to air for less than 30 seconds before weigh-in. This process had an added bonus of keeping the device secure and protected before testing as well.

I would say that if you are in a high-humidity location (or weather), it's even more important to do this. You can get a dry box to under 20% RH pretty easily and if the ambient is > 60%, you will drop closer to 3% mass. That is a huge deal especially with lighter devices.

I hadn't thought about different CA formulations. I like to use the 50 cps stuff which is pretty thin. This is pretty easy to test... just build a couple devices, pre-test them to something reasonable, put them on a shelf for a year and then test to failure. That's basically what I did with a bunch of old boomis that I had leftover from previous years.

Marc
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Re: Bridge B/C

Post by mklinger »

JonB wrote: April 13th, 2022, 5:24 am We have done silica and not done silica... to be honest, it is probably not worth your time, but it might depend on how quickly you can get your balsa structure from the "holding container" (with silica) to the loading process. If there are a few minutes of time between, it will simply re-gain the mass by absorbing moisture from the air (especially here in Florida since we have high humidity). I sometimes feel that silica/drying adds a variable that the students cannot actually know.

Also, as far as CA and weakening- I might argue that not all CA is created equally- some might become more brittle over time, others might not. This is purely conjecture- I have no evidence for this statement but so many of them have other ingredients in them in addition to the CA.

Another benefit of using the dry-box, the students should know within 0.1g exactly how much their device should weigh (in any weather conditions). We have seen bad weigh-in techniques from proctors, either with non-zeroed scales, or outright touching the device while it was on the scale!! If the students know exactly what their device should weigh, they can immediately point out any issues with the proctor. Of course, there is always a bit of variation, but if something is 0.5g off, there is a big problem and it's the only chance to get it sorted out.

Marc
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Re: Bridge B/C

Post by JonB »

mklinger wrote: April 13th, 2022, 5:40 am
JonB wrote: April 13th, 2022, 5:24 am We have done silica and not done silica... to be honest, it is probably not worth your time, but it might depend on how quickly you can get your balsa structure from the "holding container" (with silica) to the loading process. If there are a few minutes of time between, it will simply re-gain the mass by absorbing moisture from the air (especially here in Florida since we have high humidity). I sometimes feel that silica/drying adds a variable that the students cannot actually know.

Also, as far as CA and weakening- I might argue that not all CA is created equally- some might become more brittle over time, others might not. This is purely conjecture- I have no evidence for this statement but so many of them have other ingredients in them in addition to the CA.

Another benefit of using the dry-box, the students should know within 0.1g exactly how much their device should weigh (in any weather conditions). We have seen bad weigh-in techniques from proctors, either with non-zeroed scales, or outright touching the device while it was on the scale!! If the students know exactly what their device should weigh, they can immediately point out any issues with the proctor. Of course, there is always a bit of variation, but if something is 0.5g off, there is a big problem and it's the only chance to get it sorted out.

Marc

I agree with everything you said. We miss the excitement of in-person, no matter how it is being run (we only had 1 in-person competition this year). I also think that there are so many unique problem solving lessons from competing in-person when students check their devices in and run into a problem.

I think one detail that we need to focus on is that the structural design must be "on point" for silica to really matter. If there is a flaw in the design or build, silica won't save the score. I am not saying that the design needs to be perfect, it just should be the main focus. I have had students in the distant past that really, really wanted to use silica but there were so many flaws in the build that a ~3% reduction in weight would not equate to much.
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Re: Bridge B/C

Post by mklinger »

JonB wrote: April 13th, 2022, 5:56 am
mklinger wrote: April 13th, 2022, 5:40 am
JonB wrote: April 13th, 2022, 5:24 am We have done silica and not done silica... to be honest, it is probably not worth your time, but it might depend on how quickly you can get your balsa structure from the "holding container" (with silica) to the loading process. If there are a few minutes of time between, it will simply re-gain the mass by absorbing moisture from the air (especially here in Florida since we have high humidity). I sometimes feel that silica/drying adds a variable that the students cannot actually know.

Also, as far as CA and weakening- I might argue that not all CA is created equally- some might become more brittle over time, others might not. This is purely conjecture- I have no evidence for this statement but so many of them have other ingredients in them in addition to the CA.

Another benefit of using the dry-box, the students should know within 0.1g exactly how much their device should weigh (in any weather conditions). We have seen bad weigh-in techniques from proctors, either with non-zeroed scales, or outright touching the device while it was on the scale!! If the students know exactly what their device should weigh, they can immediately point out any issues with the proctor. Of course, there is always a bit of variation, but if something is 0.5g off, there is a big problem and it's the only chance to get it sorted out.

Marc

I agree with everything you said. We miss the excitement of in-person, no matter how it is being run (we only had 1 in-person competition this year). I also think that there are so many unique problem solving lessons from competing in-person when students check their devices in and run into a problem.

I think one detail that we need to focus on is that the structural design must be "on point" for silica to really matter. If there is a flaw in the design or build, silica won't save the score. I am not saying that the design needs to be perfect, it just should be the main focus. I have had students in the distant past that really, really wanted to use silica but there were so many flaws in the build that a ~3% reduction in weight would not equate to much.
Oh, absolutely! First step is a good design and good build techniques. This is what I tried to give folks a jump start with my Balsa Engineering YouTube channel this year. Second step is design tweaks and material optimization - this can/should last for the majority of the season. Finally, the weight reduction from the dry-box could actually help.

Early in the season, we'd use a dry box mainly just for device transportation and protection, not the actual weight loss.

Yes, even though my daughter is now "Division D" and I'm not directly involved in a particular team, it's a huge shame about remote competitions. Hopefully by next fall everything will be back to normal in that regard. Fingers crossed!

Marc
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Re: Bridge B/C

Post by jgrischow1 »

Great discussion. I definitely think with the 5,000 g bonus it's worth pre-testing to full load.

One thing we haven't done is test to failure. Harder to do when you don't have a custom-built dynamic load cell ;) We will try that more for next season so we have a better idea how much of a safety factor we have built in. It's possible his bridge was just barely hanging on and a second test did it in and if we had tested more to failure we would have a better idea of that model's limits.
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