Bridge B/C

[GeorgeT]

Thanks!

[mklinger]

This is a Division B design refinement and mass-reduced version of bridge #4. It uses legs made of out single pieces of wood instead of engineering angled legs. This is a slightly easier bridge to build, so it might be a good option for some teams.

The legs were made from 1/4" balsa and were cut to 8mm wide. The primary tension pieces were 1/16" x 1/16" basswood. The horizontal cross supports were 1/16" x 3/32" basswood and the "X" was 1/16" thick balsa.

The total mass was 6.98 g and it held 15.90 kg for a competition score of (15000 + 5000 bonus)/6.98 = 2865.3

https://youtu.be/JDJaApyUits

Feel free to reach out if you have any questions!

[ymlk85]

Thank you for the video, it helps me a lot. However, I do have some questions. I always have problems when cutting the balsa sheet. It comes out really bad and uneven when I used a cutting knife. What type of tool do you use to cut the sheet? Thanks in advance.

[mklinger]

Thank you for the video, it helps me a lot. However, I do have some questions. I always have problems when cutting the balsa sheet. It comes out really bad and uneven when I used a cutting knife. What type of tool do you use to cut the sheet? Thanks in advance.

Thanks! Yeah, cutting the balsa sheets can be tricky. I actually made an entire video on creating a balsa library if you want to check it out:

https://youtu.be/bLsTOpmozGI

Let me know if you have any questions, I'd be happy to try and help.

[meatz123]

Building Rules Question(s): For Bridge Level C

Hi
Does anyone know if you can do these things in this year's 2022 Bridge event:
1> bend the balsa wood
2> make notched joints (and if so, is it only certain types)
ty

[mklinger]

Here is a video where I take an experimental look at balsa density versus efficiency and what that can teach us about the Science Olympiad structural builds:

https://youtu.be/mzMFhd4SqP8

[jgrischow1]

Here is a video where I take an experimental look at balsa density versus efficiency and what that can teach us about the Science Olympiad structural builds:

https://youtu.be/mzMFhd4SqP8

Great video. Would be interested in hearing a discussion of your method of testing vs. the "single finger pushdown test" mentioned extensively in past forums.

[mklinger]

Here is a video where I take an experimental look at balsa density versus efficiency and what that can teach us about the Science Olympiad structural builds:

https://youtu.be/mzMFhd4SqP8

Great video. Would be interested in hearing a discussion of your method of testing vs. the "single finger pushdown test" mentioned extensively in past forums.

Thanks! I'm not quite sure what exactly the method you are referring to is, but I could imagine you could do a similar test using two scales on either end of the stick and pressing down in the middle if you record the maximum setting on each scale and add them together, or maybe use a single scale and multiple by two, that would give similar results to what I did.

The real key is having a nice way to keep the max setting and with using the load cell I custom built, it does just that. It works great for loading the bucket with sand so you don't need to stop the sand when it breaks which is the whole reason I built it.

I was initially worried that the accuracy at the 100-1000g range wouldn't be good enough for a test like this, but I was pleasantly surprised at the quality of data I was able to collect.

[jgrischow1]

Here is a video where I take an experimental look at balsa density versus efficiency and what that can teach us about the Science Olympiad structural builds:

https://youtu.be/mzMFhd4SqP8

Great video. Would be interested in hearing a discussion of your method of testing vs. the "single finger pushdown test" mentioned extensively in past forums.

Thanks! I'm not quite sure what exactly the method you are referring to is, but I could imagine you could do a similar test using two scales on either end of the stick and pressing down in the middle if you record the maximum setting on each scale and add them together, or maybe use a single scale and multiple by two, that would give similar results to what I did.

The real key is having a nice way to keep the max setting and with using the load cell I custom built, it does just that. It works great for loading the bucket with sand so you don't need to stop the sand when it breaks which is the whole reason I built it.

I was initially worried that the accuracy at the 100-1000g range wouldn't be good enough for a test like this, but I was pleasantly surprised at the quality of data I was able to collect.

Years ago someone on these forums (probably balsa man) described a technique called a single finger push down test in which pieces of wood of the same size are placed vertically on a scale and you literally push down on the stick until the stick starts to bend. You record how much weight it took to bend the stick and you have a proxy for stiffness. Like your test, very high correlation with density, but easy for the kids to do without any gadgetry. If you can find outliers that are quite stiff for their density those might be your winning sticks. Talked about extensively in the towers forums and then in the boomi forums as well.

[mklinger]

Great video. Would be interested in hearing a discussion of your method of testing vs. the "single finger pushdown test" mentioned extensively in past forums.

Thanks! I'm not quite sure what exactly the method you are referring to is, but I could imagine you could do a similar test using two scales on either end of the stick and pressing down in the middle if you record the maximum setting on each scale and add them together, or maybe use a single scale and multiple by two, that would give similar results to what I did.

The real key is having a nice way to keep the max setting and with using the load cell I custom built, it does just that. It works great for loading the bucket with sand so you don't need to stop the sand when it breaks which is the whole reason I built it.

I was initially worried that the accuracy at the 100-1000g range wouldn't be good enough for a test like this, but I was pleasantly surprised at the quality of data I was able to collect.

Years ago someone on these forums (probably balsa man) described a technique called a single finger push down test in which pieces of wood of the same size are placed vertically on a scale and you literally push down on the stick until the stick starts to bend. You record how much weight it took to bend the stick and you have a proxy for stiffness. Like your test, very high correlation with density, but easy for the kids to do without any gadgetry. If you can find outliers that are quite stiff for their density those might be your winning sticks. Talked about extensively in the towers forums and then in the boomi forums as well.

Ahh, yeah, that's a great way to test the onset of buckling and probably does correlate directly to density.

I liked to use that test to show how the buckling strength increases as the square of the distance decreases. You do that push test with, say a 40 cm piece, and then hold the same piece at the 20cm point and do it again to see 4x the value. This is really important for tower design and really drives how many cross supports your design needs. It also demonstrates how important it is to put glue at the middle of any stabilizing "X" joint as you are quadrupling the buckling strength!