Bridge B/C

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sneepity
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Re: Bridge B/C

Post by sneepity »

JonB wrote: September 23rd, 2021, 8:20 am
knightmoves wrote: September 23rd, 2021, 7:32 am
mklinger wrote: September 23rd, 2021, 4:55 am Like JonB mentions, you could also do a tapered design, and maybe that will be overall lighter, but it will certainly be more complex to build and it will be wider at the bottom which means more material, at least at the base connection points and that might add more mass than just using simple cross bracing on top.
I'm wondering about some sort of arch shape, although that's obviously going to be harder to build. There's a picture I've seen from a previous bridge competition, where someone built a beautiful laminated arch.

But I think your suggestion of starting with a simple design, and then using that as a benchmark to compare more "interesting" creations is a good one.
Arches are structurally very strong IF they are built perfectly- which is basically impossible since wood we use in this event has natural variation. I would stay away from the arch design. I am not saying it will not work (it can work), I am just saying I have not seen many successful arch design over the year in Sci Oly.
I feel like even an arch with cross supports will be hard. Because it requires boiling the wood and multiple other chores XD which is much easier and would have comparable results to the classic design, to be honest.
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Re: Bridge B/C

Post by mklinger »

I managed to get my Division B bridge design down to 7.52g with some straight forward material optimization. This time it held 15.28 kg for a competition score of (15000+5000)/7.52 or 2659.6!

Definitely a nice improvement with just a single iteration. That is pretty uncommon BTW, usually it takes a couple tries to get gains like that.

You can see the new build and testing of the bridge here:

https://youtu.be/cmhBslzbEFI

Compared to the previous build, the changes involved:
1. Reducing the density of the beam legs
2. Reduce the thickness and mass of the angle part of the beam from 1/16 to1/20"
(both width dimensions are still 10mm)
3. Change all the basswood components from 1/8 x 1/6 to 3/32 x 1/16. This includes the bottom tension members as well as the 3 horizontal cross-supports on each side.
4. Change to a simple "X" design for additional lateral support near the top.

This build was also slightly better built from a being level in both directions standpoint.

Feel free to reach out if you have any questions. I think this is getting close to a really nice starting design to help people who might be struggling with the big changes in the rules this year.

Thanks!

Marc
Last edited by mklinger on September 24th, 2021, 5:37 pm, edited 1 time in total.
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sneepity (September 24th, 2021, 7:03 pm) • Camponotus (September 24th, 2021, 7:34 pm) • bagman78 (April 26th, 2022, 12:32 pm)
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Re: Bridge B/C

Post by sneepity »

mklinger wrote: September 24th, 2021, 5:25 pm I managed to get my Division B bridge design down to 7.52g with some straight forward material optimization. This time it held 15.28 kg for a competition score of (15000+5000)/7.52 or 2659.6!

Definitely a nice improvement with just a single iteration. That is pretty uncommon BTW, usually it takes a couple tries to get gains like that.

You can see the new build and testing of the bridge here:

https://youtu.be/cmhBslzbEFI

Compared to the previous build, the changes involved:
1. Reducing the density of the beam legs
2. Reduce the thickness and mass of the angle part of the beam from 1/16 to1/20"
(both width dimensions are still 10mm)
3. Change all the basswood components from 1/8 x 1/6 to 3/32 x 1/16. This includes the bottom tension members as well as the 3 horizontal cross-supports on each side.
4. Change to a simple "X" design for additional lateral support near the top.

This build was also slightly better built from a being level in both directions standpoint.

Feel free to reach out if you have any questions. I think this is getting close to a really nice starting design to help people who might be struggling with the big changes in the rules this year.

Thanks!

Marc
I think I should know this, but how are you measuring density of your stick? Do you just use the cross section times height and the mass on the scale? And for scales which ones do you guys use- my kitchen scale is horribly inaccurate.
And that's a great score :D thanks for sharing Marc!!!
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Re: Bridge B/C

Post by knightmoves »

sneepity wrote: September 24th, 2021, 7:03 pm I think I should know this, but how are you measuring density of your stick? Do you just use the cross section times height and the mass on the scale? And for scales which ones do you guys use- my kitchen scale is horribly inaccurate.
And that's a great score :D thanks for sharing Marc!!!
You can buy small scales that measure to "0.01g" (the last digit is a bit optimistic, it's more like one-and-a-half decimal places) for $10 or so on amazon. They're marketed for use for jewelry or "herbs". 100g max weight should be fine.
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Re: Bridge B/C

Post by mklinger »

sneepity wrote: September 24th, 2021, 7:03 pm
mklinger wrote: September 24th, 2021, 5:25 pm I managed to get my Division B bridge design down to 7.52g with some straight forward material optimization. This time it held 15.28 kg for a competition score of (15000+5000)/7.52 or 2659.6!

Definitely a nice improvement with just a single iteration. That is pretty uncommon BTW, usually it takes a couple tries to get gains like that.

You can see the new build and testing of the bridge here:

https://youtu.be/cmhBslzbEFI

Compared to the previous build, the changes involved:
1. Reducing the density of the beam legs
2. Reduce the thickness and mass of the angle part of the beam from 1/16 to1/20"
(both width dimensions are still 10mm)
3. Change all the basswood components from 1/8 x 1/6 to 3/32 x 1/16. This includes the bottom tension members as well as the 3 horizontal cross-supports on each side.
4. Change to a simple "X" design for additional lateral support near the top.

This build was also slightly better built from a being level in both directions standpoint.

Feel free to reach out if you have any questions. I think this is getting close to a really nice starting design to help people who might be struggling with the big changes in the rules this year.

Thanks!

Marc
I think I should know this, but how are you measuring density of your stick? Do you just use the cross section times height and the mass on the scale? And for scales which ones do you guys use- my kitchen scale is horribly inaccurate.
And that's a great score :D thanks for sharing Marc!!!
I don't actually bother calculate the actual density (very often), but I just mean lowering or raising the relative density by picking a part with the same dimensions that has a lower or higher mass.
We were fortunate to have a very nice milligram accurate scale for the past couple years. This one in particular:

https://www.amazon.com/Ohaus-SPX123-Por ... 823&sr=8-3

but that is very expensive and absolutely not required.

For about $15 you can get a scale like this which should be fine and much, much better than your normal kitchen scale:

https://www.amazon.com/GDEALER-Digital- ... E3A0&psc=1

For any scale, make sure it's on a firm, flat surface or you might get bad readings.
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Re: Bridge B/C

Post by sneepity »

mklinger wrote: September 25th, 2021, 7:48 am
sneepity wrote: September 24th, 2021, 7:03 pm
mklinger wrote: September 24th, 2021, 5:25 pm I managed to get my Division B bridge design down to 7.52g with some straight forward material optimization. This time it held 15.28 kg for a competition score of (15000+5000)/7.52 or 2659.6!

Definitely a nice improvement with just a single iteration. That is pretty uncommon BTW, usually it takes a couple tries to get gains like that.

You can see the new build and testing of the bridge here:

https://youtu.be/cmhBslzbEFI

Compared to the previous build, the changes involved:
1. Reducing the density of the beam legs
2. Reduce the thickness and mass of the angle part of the beam from 1/16 to1/20"
(both width dimensions are still 10mm)
3. Change all the basswood components from 1/8 x 1/6 to 3/32 x 1/16. This includes the bottom tension members as well as the 3 horizontal cross-supports on each side.
4. Change to a simple "X" design for additional lateral support near the top.

This build was also slightly better built from a being level in both directions standpoint.

Feel free to reach out if you have any questions. I think this is getting close to a really nice starting design to help people who might be struggling with the big changes in the rules this year.

Thanks!

Marc
I think I should know this, but how are you measuring density of your stick? Do you just use the cross section times height and the mass on the scale? And for scales which ones do you guys use- my kitchen scale is horribly inaccurate.
And that's a great score :D thanks for sharing Marc!!!
I don't actually bother calculate the actual density (very often), but I just mean lowering or raising the relative density by picking a part with the same dimensions that has a lower or higher mass.
We were fortunate to have a very nice milligram accurate scale for the past couple years. This one in particular:

https://www.amazon.com/Ohaus-SPX123-Por ... 823&sr=8-3

but that is very expensive and absolutely not required.

For about $15 you can get a scale like this which should be fine and much, much better than your normal kitchen scale:

https://www.amazon.com/GDEALER-Digital- ... E3A0&psc=1

For any scale, make sure it's on a firm, flat surface or you might get bad readings.
Thank you! yeah it makes sense!
yep and I just ordered a scale. also ty knightmoves :3
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Re: Bridge B/C

Post by mklinger »

As you guys probably already know, Division C is much harder than Division B this year due to the pass-through block being significantly larger and the width being wider than the loading block itself.

I briefly took a look at a simple scaled up version of my Division B design, but I wanted to try a tapered design to see how it would handle the design parameters.

I was able to build a 12.53g bridge that held 15.84kg for a competition score of 1596.2 ((15000 + 5000 bonus)/12.53).

While I doubt that score will win any major competitions, the goal was to give you guys a starting point to see how a tapered design works and could be built.

In my video, I not only show the testing and pretty cool failure in slow-mo, but also a bunch of steps along the way on exactly how I built it.

https://youtu.be/tvfrtC_1W58

I hope this helps some of the Division C students just starting out!

Marc
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Re: Bridge B/C

Post by sneepity »

mklinger wrote: October 3rd, 2021, 3:22 pm As you guys probably already know, Division C is much harder than Division B this year due to the pass-through block being significantly larger and the width being wider than the loading block itself.

I briefly took a look at a simple scaled up version of my Division B design, but I wanted to try a tapered design to see how it would handle the design parameters.

I was able to build a 12.53g bridge that held 15.84kg for a competition score of 1596.2 ((15000 + 5000 bonus)/12.53).

While I doubt that score will win any major competitions, the goal was to give you guys a starting point to see how a tapered design works and could be built.

In my video, I not only show the testing and pretty cool failure in slow-mo, but also a bunch of steps along the way on exactly how I built it.

https://youtu.be/tvfrtC_1W58

I hope this helps some of the Division C students just starting out!

Marc
Hey Marc, it looks great! :) I had a quick question.. how did you cut the angles for the main four truss members when they're angled in and they touch the ground at an angle? Thanks so much.
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Re: Bridge B/C

Post by mklinger »

sneepity wrote: October 4th, 2021, 4:40 am
mklinger wrote: October 3rd, 2021, 3:22 pm As you guys probably already know, Division C is much harder than Division B this year due to the pass-through block being significantly larger and the width being wider than the loading block itself.

I briefly took a look at a simple scaled up version of my Division B design, but I wanted to try a tapered design to see how it would handle the design parameters.

I was able to build a 12.53g bridge that held 15.84kg for a competition score of 1596.2 ((15000 + 5000 bonus)/12.53).

While I doubt that score will win any major competitions, the goal was to give you guys a starting point to see how a tapered design works and could be built.

In my video, I not only show the testing and pretty cool failure in slow-mo, but also a bunch of steps along the way on exactly how I built it.

https://youtu.be/tvfrtC_1W58

I hope this helps some of the Division C students just starting out!

Marc
Hey Marc, it looks great! :) I had a quick question.. how did you cut the angles for the main four truss members when they're angled in and they touch the ground at an angle? Thanks so much.
Hi sneepity, thanks!! Do you mean how did I figure out the angle, or how did I actually cut it? For figuring out the angle, I just used scrap wood and did some trial and error. That seems like the best option.
For actually cutting it, I used a miter cutter which you can occasionally see in my videos. This is another extremely handy tool for this event. I only use the X-Acto knife for things like cutting thin/small balsa parts. For all basswood cutting and larger balsa, I always use the miter as it makes a much cleaner cut.

It looks like it's not available on Amazon anymore, but it does looks like you can get it directly from the manufacturer:

http://www.fourmostproducts.com/our-pro ... ion-tools/

Oh, I just re-read your question... the angle of the primary part of the beams touching the ground. Basically, I didn't bother worry about that at all. I probably would have made it worse by trying to sand them to the correct angle. Balsa is pretty strong such that it can be loaded along an edge like that and the load will still be distributed through the stick. Because the bridge is being loaded evenly across all 4 legs (theoretically), all those legs will get compressed in the same fashion. I just re-checked the bridge after testing, and all 4 legs at the bottom still had their angle, so they didn't even get compressed flat!
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Re: Bridge B/C

Post by mklinger »

One other quick comment on that miter cutter. It is a fantastic tool, but you have to be EXTREMELY careful with it as it is incredibly sharp. You need to make sure you never leave it in the "up" position or you can get cut by just accidently brushing up against it, grabbing it, etc. The only minor accidents any of my students ever had were due to that miter cutter. Please be careful!
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