Bridge Weight - Division B

[VansBuilders]

I am new to science Olympiad. I am doing Bridge Building in Division B.
I built a prototype last wkend. See picture. I used balsa wood & super glue.
The truss part has .5 cm balsa & .3 cm balsa wood. It weighs 19.8 grams. I have not tested the load yet.
From this forum I understand that the ideal weight should ~6 grams.
1. Any thoughts on how I can bring the weight down?
2. Should I be using only .3cm balsa for all of the members & nodes?
3. Should I be using a different adhesive?

FullSizeRender (1).jpg (35.19 KiB) Viewed 6525 times

[bernard]

I am new to science Olympiad. I am doing Bridge Building in Division B.
I built a prototype last wkend. See picture. I used balsa wood & super glue.
The truss part has .5 cm balsa & .3 cm balsa wood. It weighs 19.8 grams. I have not tested the load yet.
From this forum I understand that the ideal weight should ~6 grams.
1. Any thoughts on how I can bring the weight down?
2. Should I be using only .3cm balsa for all of the members & nodes?
3. Should I be using a different adhesive?

FullSizeRender (1).jpg

Quick response since I'm doing homework right now, might come back later...

Weight isn't particularly important for you until you've tested it and know it will hold more than 15 kgs. If it doesn't hold all 15 kgs, decreasing your weight doesn't necessarily help your score. If it holds more than 15 kgs, you'd want to decrease your bridge's weight because it's able to support more than it needs to.

But even if it doesn't hold all 15 kgs, as you've probably already noticed 19.8 g is relatively heavy, compared to a lot of the competitive bridges you'll see/hear about at competitions. As you test your bridge, you'll notice some parts just never break. Maybe they're oversupported? For example, I think the members under tension (the two long sticks touching the table) are overdone. I had decent results last year with 1/16" square balsa tension.

Super glue/cyanoacrylate is what a lot of teams use. It's great because it's light, strong, and dries fast. I've tried weldbond once and never went back, mainly because waiting for it to dry is boring! Here making a lot of bridges and making smart changes helps get good results. And drying time takes away from time to make bridges.

Also I notice that the two tension members (touching the table) aren't attached with the best joint - notice how little contact there is between the wood. Put those two on the sides of the bridge and you'll have more contact, very likely a stronger joint and bridge too.

[VansBuilders]

Thanks for the feedback .I am just posting an update.
The first design was able to bear only 7.5kg only. The loading block fell through the member on the top.
We have modified the design. We made sure the top member is not wider than the loading block.
We added some side supports. We will test this design next wk.

The key question are the following:
While performing the load test, should we add the sand slowly in steps or the full quantity of sand that would equal 15kg (along with bucket +Chain + loading block assembly) all at once?
Is there a minimum time limit on how long the bridge should hold the weight 15kg?
Any feedback would be much appreciated.

[VansBuilders]

Picture of second design.

IMG_1131 (2).JPG (30.42 KiB) Viewed 6412 times

[bernard]

Thanks for the feedback .I am just posting an update.
The first design was able to bear only 7.5kg only. The loading block fell through the member on the top.
We have modified the design. We made sure the top member is not wider than the loading block.
We added some side supports. We will test this design next wk.

The key question are the following:
While performing the load test, should we add the sand slowly in steps or the full quantity of sand that would equal 15kg (along with bucket +Chain + loading block assembly) all at once?
Is there a minimum time limit on how long the bridge should hold the weight 15kg?
Any feedback would be much appreciated.

By the rules, you'll have to set up and load your sand in under 6 minutes. And from my experience, adding the sand very slowly hasn't been shown to make a bridge more efficient. In fact, supporting the load longer may weaken it.

[JonB]

Thanks for the feedback .I am just posting an update.
The first design was able to bear only 7.5kg only. The loading block fell through the member on the top.
We have modified the design. We made sure the top member is not wider than the loading block.
We added some side supports. We will test this design next wk.

The key question are the following:
While performing the load test, should we add the sand slowly in steps or the full quantity of sand that would equal 15kg (along with bucket +Chain + loading block assembly) all at once?
Is there a minimum time limit on how long the bridge should hold the weight 15kg?
Any feedback would be much appreciated.

By the rules, you'll have to set up and load your sand in under 6 minutes. And from my experience, adding the sand very slowly hasn't been shown to make a bridge more efficient. In fact, supporting the load longer may weaken it.

Agreed about loading quickly. Load as quickly as possible ASSUMING that the loading process remains smooth.

[Unome]

Picture of second design.

IMG_1131 (2).JPG

Just a sidenote: if this had competed and the event supervisor had been paying attention to the FAQs, it would have likely been Tier 3'ed for the loading block not aligning with the centerline (or at least I think it doesn't; it might just be the angle of the picture). Of course, not all supervisors will know the FAQs, and some that do might just let it slip, but going off of something Z said last year (or maybe several years ago), I'd say the risk isn't worth it (although I'm not sure whether keeping the bridge off-center would actually help it support the load; my knowledge of Bridges is limited and very specific).

That was a long sidenote

[JonB]

Picture of second design.

IMG_1131 (2).JPG

Just a sidenote: if this had competed and the event supervisor had been paying attention to the FAQs, it would have likely been Tier 3'ed for the loading block not aligning with the centerline (or at least I think it doesn't; it might just be the angle of the picture). Of course, not all supervisors will know the FAQs, and some that do might just let it slip, but going off of something Z said last year (or maybe several years ago), I'd say the risk isn't worth it (although I'm not sure whether keeping the bridge off-center would actually help it support the load; my knowledge of Bridges is limited and very specific).

That was a long sidenote

I was wondering about this. I am not sure how to ask this, but is it referring to the center line on the x or y axis? I am not sure how else to phrase this. Center line of the width OR length of the bridge? Maybe that is better.

[bernard]

Picture of second design.

Just a sidenote: if this had competed and the event supervisor had been paying attention to the FAQs, it would have likely been Tier 3'ed for the loading block not aligning with the centerline (or at least I think it doesn't; it might just be the angle of the picture). Of course, not all supervisors will know the FAQs, and some that do might just let it slip, but going off of something Z said last year (or maybe several years ago), I'd say the risk isn't worth it (although I'm not sure whether keeping the bridge off-center would actually help it support the load; my knowledge of Bridges is limited and very specific).

That was a long sidenote :)

I was wondering about this. I am not sure how to ask this, but is it referring to the center line on the x or y axis? I am not sure how else to phrase this. Center line of the width OR length of the bridge? Maybe that is better.

I see two rules, 3.e. and 5.b.iv.. When considered together, the rules seem specify a location for the loading block both along the Bridge span and along the test base. 3.e., the rule that specifies a location along the Bridge span has it at the center (along the length). 5.b.iv., the rule that specifies a location relative to the test base could be interpreted as precisely in the center as possible - centered both in the directions of the width and length of the bridge. And it seems possible for a setup to follow one rule but not the other.

[JonB]

Just a sidenote: if this had competed and the event supervisor had been paying attention to the FAQs, it would have likely been Tier 3'ed for the loading block not aligning with the centerline (or at least I think it doesn't; it might just be the angle of the picture). Of course, not all supervisors will know the FAQs, and some that do might just let it slip, but going off of something Z said last year (or maybe several years ago), I'd say the risk isn't worth it (although I'm not sure whether keeping the bridge off-center would actually help it support the load; my knowledge of Bridges is limited and very specific).

That was a long sidenote

I was wondering about this. I am not sure how to ask this, but is it referring to the center line on the x or y axis? I am not sure how else to phrase this. Center line of the width OR length of the bridge? Maybe that is better.

I see two rules, 3.e. and 5.b.iv.. When considered together, the rules seem specify a location for the loading block both along the Bridge span and along the test base. 3.e., the rule that specifies a location along the Bridge span has it at the center (along the length). 5.b.iv., the rule that specifies a location relative to the test base could be interpreted as precisely in the center as possible - centered both in the directions of the width and length of the bridge. And it seems possible for a setup to follow one rule but not the other.

I can definitely see how event supervisors could struggle interpreting/implementing the wording of those specific rules. Fantastic diagrams- thank you Bernard!