Designs B/C

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Re: Designs B/C

Postby iwonder » Mon Mar 16, 2015 9:26 pm

SO_2015 wrote:Hi,

I have a question related to testing the bridge before the event.

Lets say ,the 'final'(the one which I will bring to the event) bridge can hold 14kg of weight . So before bringing it to the event, with what weight should I test the bridge ?

Is it 7kg(half of weight which it can handle) or full 14 kg(which I don't feel comfortable) or some other weight enough to test the strength?

thanks


If you want to test your bridge (no one says you have to, but it's saved me a few embarrassing moments). Then easing it up to about halfway like chinesesushi said is a good benchmark. Listen for any major creaks or pops as it loads (make sure it's really quiet in the room), these could be signs that it won't hold as much load at the contest. Some small noises aren't too bad, since that's mostly just the bridge 'settling' (which is actually a good thing). And of course if it fails when you put the bucket on because you forgot to glue a joint... well now you can fix it. (unlike me, at the contest... whoops).
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Re: Designs B/C

Postby SO_2015 » Mon Mar 16, 2015 9:36 pm

Thank you .

Like you mentioned, I want to avoid the 'surprise' at event but was debating between that and making bridge weak.

Not to take liberty but may I ask you( and other Guru's) one more question- it seems one way to lighten the bridge is to sand it. In my experience sanding a bridge has it made it weak. Is there a trick to sanding or it is the common experience.

Thanks for taking time out to help.

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Re: Designs B/C

Postby iwonder » Mon Mar 16, 2015 9:39 pm

SO_2015 wrote:Thank you .

Like you mentioned, I want to avoid the 'surprise' at event but was debating between that and making bridge weak.

Not to take liberty but may I ask you( and other Guru's) one more question- it seems one way to lighten the bridge is to sand it. In my experience sanding a bridge has it made it weak. Is there a trick to sanding or it is the common experience.

Thanks for taking time out to help.


It's just a matter of where you're sanding. If you sand on the members then yes, it will get weaker, if you choose the right portions of joints then you'll be just fine. It might save you 0.1-0.3g depending on densities and how your joints are formed.
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Re: Designs B/C

Postby SO_2015 » Tue Mar 17, 2015 2:46 am

Thank you.

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Re: Designs B/C

Postby SO_2015 » Fri Mar 20, 2015 5:31 am

I have another question related to the 'outside width' equals to 5 cm.
In my case I am using balsa wood which has a thickness of 1.2 cm (05.inch). This piece of wood is on each side of the bridge.
So, when we calculate 5 cm width, should we include this to calculate the 5 cm or just the 'empty' space between the two sides should be 5 cm.
In latter case, my end to end width would become , 7.4 cm. Which one is correct?

Thanks

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Re: Designs B/C

Postby bernard » Fri Mar 20, 2015 6:05 am

SO_2015 wrote:I have another question related to the 'outside width' equals to 5 cm.
In my case I am using balsa wood which has a thickness of 1.2 cm (05.inch). This piece of wood is on each side of the bridge.
So, when we calculate 5 cm width, should we include this to calculate the 5 cm or just the 'empty' space between the two sides should be 5 cm.
In latter case, my end to end width would become , 7.4 cm. Which one is correct?

Thanks

The 7.4cm is correct. Just to make sure I'm understanding you correctly: the orange in the image below is included in the width measurement.

Image
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Re: Designs B/C

Postby SO_2015 » Fri Mar 20, 2015 6:30 am

Thank you Bernard.

My use case is second type or middle one, per your picture.
The block sits on the wood on either side, but is not overshooting the boundary. So by your illustration , I should be ok. right?

So, in the above scenario, the 'empty space between the two wood edges( depicted by orange color) would be 5 cm - 2*(thickness/width of wood).

I am just trying to understand - is the 5 cm from Orange to orange OR between Orange ?

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Re: Designs B/C

Postby SO_2015 » Fri Mar 20, 2015 6:35 am

Sorry... I may have jumped to conclusion...

I read your response again. ... I think you have said that 'WOOD aka Orange' is included in the width.

Maybe .. I could have phrased my question better-- Does wood width count towards 5 cm?

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Re: Designs B/C

Postby bernard » Fri Mar 20, 2015 7:18 am

SO_2015 wrote:Sorry... I may have jumped to conclusion...

I read your response again. ... I think you have said that 'WOOD aka Orange' is included in the width.

Maybe .. I could have phrased my question better-- Does wood width count towards 5 cm?

Yes, wood width counts towards 5cm. The orange is wood, and the orange is included in the measurement, which is why the second illustration in the image is allowed (with a 5.0cm loading block, the empty space between the wood would be less than 5.0cm).
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Re: Designs B/C

Postby Azn » Sun Mar 22, 2015 9:08 pm

Obviously everyone is entitled to their own opinion, and a great thing about an event like bridges is that there isn't one single sure-fire way to succeed.
When it comes to pre-testing, i've always been against it, believing that even testing to just 50% would compromise the efficiency of the device. One important challenge in the event was being able to build consistently to avoid "surprises", and testing each device only once (to failure). Then again, I understand that some people like to make sure their devices can "settle in" before the real test, so I suppose there is a level of personal preference.

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Re: Designs B/C

Postby dholdgreve » Thu Mar 26, 2015 3:09 pm

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

Postby SO_2015 » Thu Mar 26, 2015 5:12 pm

Thank you.

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Re: Designs B/C

Postby sjwon3789 » Sun Apr 12, 2015 4:34 pm

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

Postby dholdgreve » Mon Apr 13, 2015 5:39 pm

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

Postby sjwon3789 » Mon Apr 20, 2015 5:15 am

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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