Elevated Bridge B/C

[Littleboy]

The humidity in the air made my bridge go from 12.5 grams to 13.13 grams. It is the same design I used at regionals that weighed 12.2 grams and held 13 kg. But this bridge weighed more, held only 7.8 kg, and was better built! I have no clue what happened. Why did it do this

[winneratlife]

The humidity in the air made my bridge go from 12.5 grams to 13.13 grams. It is the same design I used at regionals that weighed 12.2 grams and held 13 kg. But this bridge weighed more, held only 7.8 kg, and was better built! I have no clue what happened. Why did it do this

I'm assuming you used balsa. Balsa has variable densities, so you may have ended up with higher density balsa where you didn't need it, and lower where you NEEDED higher density. I had a 300 bridge. I just changed a couple densities, and the eff more than doubled.

[Balsa Man]

JimY, it is always a pleasure when you chime in. Always learn something. Specific input and insights from a Nationals winner really adds to the value of this board. Thank you.

To help increase that value- a couple comments, and a couple questions back:

Just as I noted that “utterly amazing” was a subjective term, so is my use of the term “magic.” By magic in this case, I meant, a) it works, b) it simultaneously takes into account a number of.....factors, or variables, and deals with them in ways that work together, and c) exactly what those factors are, and how they've been addressed is not.....fully known, obvious, or understood. What is magic to someone struggling to get to an efficiency of 500 is different to someone working at 1500 and looking at 2500.

You say,”By precise building if you mean perfect angles and great workmanship, then I disagree. If you mean great glue joints and just average workmanship, then I agree.”

When I say precise building, a) again, we have a subjective term, and b) in using it, I'm talking about two aspects. The first is the joints – right amount of glue- just enough, and no gaps- a “precise” fit. The second is the.....I'll use two terms: “geometrical precision” and “symmetry.” We have all seen .....seriously imprecise bridges- one side visibly lower than the other; leg span on one side a cm larger than on the other; leg lengths different enough that it teeter-totters on two legs. They tip over or break at low loads.

In my experience, bridges that get to 1000 are .....what words to use....pretty darn precise, and symmetrical. The two sides (before assembly) if put together, would line up very closely – the alignment and length of the members, and the centers/locations of the nodes. The assembly of the sides into a bridge is likewise “precise”; the leg contact points are a rectangle; if the sides are leaned in, the amount of lean-in is very close to equal; if they're vertical, they're both very close to vertical; ladder pieces are perpendicular to the members they join, etc.

Why is this important? Because of how load distributes in a structure. If a bridge (for discussion purposes) is “perfect”- both sides are congruent- the same, and they are assembled symmetrically, then the loads – looking at any individual member and the nodes it joins (i.e., its joints), whether there are 2 or 4 of those members in the structure, are the same. As lengths and....geometrical locations of the nodes (x-y-z coordinates) move away from “perfect”/symmetrical, the load on one of a given set of (2 or 4 corresponding) pieces will increase over that seen by its corresponding pieces. If the pieces have been selected to match in strength, and we're “pushing the limits”, where they're “just strong enough to hold”, the one seeing higher load because of assymetry will fail first, or one of its joints will fail. Or, if the structure is less than symmetrical, one piece will distort, leading to distortion elsewhere, or joint strain, or both. Whatever the mode, as you note, when it starts to happen, it happens fast.

“Perfect angles- only in the context of joints involving an angled piece, where as as you note, they have to have no gaps. To do that, you have to have the angle of the piece going into the joint very close- very precise – maybe not perfect, but pretty darn close. Does it matter if the legs run at 60.05 degrees, no; does it matter that all 4 run at very close to the same angle, yes, I would say it does.
Great workmanship vs average? We're back to subjectivity and semantics; the average across all S-O bridges is a lot different than that of, say those that medal at the State level. To those struggling to reach 750, both the quality of joints you're talking about, and the level of precision/symmetry I'm talking about are “great.” To those working at the 12-1500 range, they're “average”- just part of what it takes to be there.
Last, “special” pieces of wood. A lot of weighing and testing tells me that all sticks (or strips)of a given density are not created equally – if you take a couple 36” sticks that weigh the same, and cut each into, say 10 pieces, you're going to see 10, even 20% difference in weight, and similar variation in strength. If you get 10 pieces that weigh the same- say to 1/100th of a gr, or even closer, they are going to vary in strength by ~10%; sometimes more- though usually you can see differences in the outliers. So, “special” to me means closely matched in weight, and all above the.....”strength threshold” you need - and avoiding including any of the “low outliers.

So, I think we see the same things as important; and are just using different words, but I'm not 100% sure. Love to hear more if you think otherwise. I've learned a lot over the years, but I know I don't have all the answers; learn something new every year, and can't speak from the perspective of winning Nationals.
Do you agree that precision as I've discussed it is an important factor- that the more precise, all other factors being equal, the better efficiency you'll see? I'm guessing that you guys are building to a precision of .... well inside a millimeter – if you lay 2 sides on top of each other, everything will line up that closely- am I wrong?

Thanks again for sharing your insights; very best of luck at Nats- to you, and to all!

[Littleboy]

The humidity in the air made my bridge go from 12.5 grams to 13.13 grams. It is the same design I used at regionals that weighed 12.2 grams and held 13 kg. But this bridge weighed more, held only 7.8 kg, and was better built! I have no clue what happened. Why did it do this

I'm assuming you used balsa. Balsa has variable densities, so you may have ended up with higher density balsa where you didn't need it, and lower where you NEEDED higher density. I had a 300 bridge. I just changed a couple densities, and the eff more than doubled.

How do you know where to put what kind of density balsa besides testing alot

[blue cobra]

The humidity in the air made my bridge go from 12.5 grams to 13.13 grams. It is the same design I used at regionals that weighed 12.2 grams and held 13 kg. But this bridge weighed more, held only 7.8 kg, and was better built! I have no clue what happened. Why did it do this

I'm assuming you used balsa. Balsa has variable densities, so you may have ended up with higher density balsa where you didn't need it, and lower where you NEEDED higher density. I had a 300 bridge. I just changed a couple densities, and the eff more than doubled.

How do you know where to put what kind of density balsa besides testing alot

Kindly ask Balsa Man for his data. Then test a lot to see if it will work. Before you know strength for densities you need to know strength needed, which is where programs such as JHU's bridge designer at the least come into play.

Now an update from States- as I fear what my coaches would do if I posted a picture of my bridge, I won't be posting a picture. However I have posted a screenshot from JHU's bridge designer before, so you can sort the thread by author and find it if you want to see my general design, but note that that was not my final design- I made some minute changes that had a pretty positive effect on the bridge. The compression members and those lowest stress "tension" members (testing proved that since my feet didn't slide out much, they were actually in light compression) were made of 3/32 bass- ranging from nearly 1.50g/24" stick to about 1.20g/24" stick. The tension members were paired and made of 1/8x0.020" bass. I don't really remember what weights I used for those. I had a lean in of 1/2 cm on each side and my cross members were made of light 1/32 balsa. Final mass was just shy of 8 grams and held a little over 2/3 of the weight, which got an efficiency I believe in the 1300s. I got a medal.

JimY, I'd be interested to know what size bass your students use and still get incredibly low bridge masses. However if you'd rather keep this to yourself, I would understand.

[nejanimb]

Congrats, BlueCobra. Did your team make nationals?

[blue cobra]

Thank you, and although we improved from last year, we unfortunately did not make it to Nationals. New York certainly isn't the easiest state to win, and so I am happy with our top ten finish. Hopefully next year I can make it with our high school team.

[nejanimb]

Very nice. It wouldn't be okay to post pictures even though your season is over though? Strange. Ah well - congrats!

[blue cobra]

I haven't directly asked, actually, but seeing as the C coach had the suspicious shifty eyes and told me to go pack up my bridge immediately after testing at regionals so no one could see it, as well as how they pack all their building events in opaque boxes and have even begun to play mind games on the other teams ( ) I doubt they'd look too kindly upon finding a picture of my bridge on here. And thanks again!

[lllazar]

I haven't directly asked, actually, but seeing as the C coach had the suspicious shifty eyes and told me to go pack up my bridge immediately after testing at regionals so no one could see it, as well as how they pack all their building events in opaque boxes and have even begun to play mind games on the other teams ( ) I doubt they'd look too kindly upon finding a picture of my bridge on here. And thanks again!

Hehe...congrats on ur medal At Illinois states, the highest B division eff was like 1567, #2 was like 650? So ya...not every competitive. Though i have to say the number one team had an all bass design, i actually watched them and their bridge broke when there was like a handful of sand left in the hopper...i heard they tested A LOT...