Elevated Bridge B/C

[JimY]

From BalsaMan (abbreviated from page 71): "JimY, a couple comments:

"When I say precise building, 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 geometrical precision and symmetry. We have all seen seriously imprecise bridges, such as 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. "

Yes, I agree. I consider this incredibly important, but issues with both are easily remedied with a truss pattern that is carefully checked before the first piece of wood is taped down to it. So, when we make truss patterns, after the node points are marked, we check several diagonals that span across the bridge (such as from node 2 to 9, etc). My rule of thumb is that each length should match up with its mirror image by not more than 1 mm. This step is completed in September or October, and those patterns are used for the entire season.

The second thing we do is work on getting our main compression joints to fit together with no gaps on the entire surface of the joint. This is actually easier and easier to do as you use flatter and flatter pieces of wood. So, square is not the way to go. The compression pieces later have a perpendicular piece added to make them L shaped. This gives higher strength in compression than square pieces of the same total cross sectional area. Two reasons to not go square.

Regarding the wood, since the bass used is much more consistent on a percentage basis than balsa, we don't go to the point of weighing sticks. For the small amount of balsa used, first, I try to purchase sheets that appear to have consistent density throughout the sheet. For that, I just hold it up to the light in the store. If it looks like a zebra, I leave it at the store. Then I weigh the sheets purchased and mark them. So each strip cut from a sheet will have somewhat limited variability. I don't weigh the strips cut from these balsa sheets either. I did bass stick weighing when boom was a C event a couple years ago, but just about drove myself nuts. We earned a 7th and a 5th at nats in the event those two seasons, but weighing the bass for that event in the end didn't matter that much because we didn't do enough building to know what density we actually needed. I'm not saying that weighing isn't useful, but rather that we don't do it.

Somebody else asked what wood dimensions we use. All I can say is that except for the smallest pieces, nothing is square. Flat gives the advantages mentioned here, plus greater glue surface area, which I mentioned a page or so back, for things like lap joints and gussets.

I hope this helps the teams that are competing at nats.

[Balsa Man]

Blue cobra, thanks for the pics; very nice work!
JimY, thanks for the additional insights; very interesting and useful.

[packer-backer91]

I am going to concur that humidity alone should not be a major factor in underperformance.....

Very good analysis
Agree.

I'm not sure that we really understand what is happening with humidity and bridges scores. Yes other than state I don't have a lot of data on the effect of humidity but it 100% sure it does have some effect. The odds of the top 6 bridges underachieving is proof that it has some effect. My schools bridge did as I think a little better score at Regional's than their score at state. That should never happen the Reg bridge was built in a few hours the night for before [I don't know if any of it was rushed or not]. The state bridge was a extremely well built they spent a few days building a few bridges that where perfectly built from hand selected and weighted peaces of wood. This bridges should have been at least as high as the reg one they built. Troy had the 2000 plus score last year did not come close to that score at state, but I bet they where more disappointed with lower score and placing then the weather did have some effect on the score. The only way to really find the answer to this question is to take two identical peaces of wood [weight/density/mass] then brake one at one set of weather then break another stick with different weather I bet that the scores would had a big difference.

[Balsa Man]

[

I'm not sure that we really understand what is happening with humidity and bridges scores. Yes other than state I don't have a lot of data on the effect of humidity but it 100% sure it does have some effect. The odds of the top 6 bridges underachieving is proof that it has some effect. ........The only way to really find the answer to this question is to take two identical peaces of wood [weight/density/mass] then brake one at one set of weather then break another stick with different weather I bet that the scores would had a big difference.

Some effect, yeah, maybe. A "big" difference? I don't think so.
As I posted, we did exactly what you're suggesting - testing of a set of pieces (that while not "identical", were pretty darn close) in both dry and humid conditions, and there was no discernable difference. Stamping Kid is reporting the same sort of findings testing a whole structure. Were all other variables tightly controlled in either case? No, so I'm not saying this is conclusive proof, but it is real data around the question. Likewise the "underachievement" you note is not "proof" of the converse. Again, I note, as I did, that a significant factor could be damage from pre-testing

[nejanimb]

Also, if we want to talk about how conditions could negatively impact scores... last year's nationals had just about the worst testing conditions possible (well, the setup was great, but I mean in terms of weather and location), and scores were still very high. The Georgia air was practically wet, and the testing location was on a stage in an open gym with hundreds of other people in there (because Robots and EV were also being run in the same gym), but several teams in both divisions broke 1500, while a few broke 2,000. Though I can see how it might have some effect, that's almost definitely not the main cause for issue. There are lots of other reasons bridges can underperform (unfortunately).

BlueCobra, thanks for the pictures. Looks like very good work. I also totally second what JimY said about square pieces... For anything above 1/16th square, I'm totally opposed to square cross sections! Harder to work with, and I don't think its as successful.

[Littleboy]

NEVER use square pieces. They add weight where it is not needed.

Couldn't humidity warp the wood to make it bend and break at low loads?

[blue cobra]

NEVER use square pieces. They add weight where it is not needed.

Couldn't humidity warp the wood to make it bend and break at low loads?

I'll certainly experiment with non-square pieces next season, but square pieces have enough of a place that I wouldn't say to never use them. They're easier to work with, and you don't get added glue weight, although a lamination, done correctly, makes up for its weight. By "They add weight where it is not needed." are you saying that a lamination can be more efficient (which is true). If not, what did you mean?

Humidity can weaken the wood some, but not enough to have a major effect. Maybe something like 1500 to 1450, but not 1500 to 1000.

[Littleboy]

NEVER use square pieces. They add weight where it is not needed.

Couldn't humidity warp the wood to make it bend and break at low loads?

I'll certainly experiment with non-square pieces next season, but square pieces have enough of a place that I wouldn't say to never use them. They're easier to work with, and you don't get added glue weight, although a lamination, done correctly, makes up for its weight. By "They add weight where it is not needed." are you saying that a lamination can be more efficient (which is true). If not, what did you mean?

Humidity can weaken the wood some, but not enough to have a major effect. Maybe something like 1500 to 1450, but not 1500 to 1000.

If you have an 1/8 inch square piece and a 1/8 by 1/16 piece the 1/8 by 1/16 piece of wood would be lighter and about as strong.

[blue cobra]

NEVER use square pieces. They add weight where it is not needed.

Couldn't humidity warp the wood to make it bend and break at low loads?

I'll certainly experiment with non-square pieces next season, but square pieces have enough of a place that I wouldn't say to never use them. They're easier to work with, and you don't get added glue weight, although a lamination, done correctly, makes up for its weight. By "They add weight where it is not needed." are you saying that a lamination can be more efficient (which is true). If not, what did you mean?

Humidity can weaken the wood some, but not enough to have a major effect. Maybe something like 1500 to 1450, but not 1500 to 1000.

If you have an 1/8 inch square piece and a 1/8 by 1/16 piece the 1/8 by 1/16 piece of wood would be lighter and about as strong.

Assuming all other factors constant, that is not really true. Please correct me if I'm wrong, but I believe the "theoretical" (ignoring wood inconsistencies) tensile strength would be half, and I think the compressive strength would be less than half, since there is less wood and it would be more prone to buckling.

[smartkid222]

to contribute to the humidity discussion:
this website was posted on last year's thread
http://www.globalspec.com/reference/725 ... n-ProBalsa
you can also search "humidity" on last year's thread and find some people's opinions and espeically a lot of talk about nationals.
One example was the warping effect the humidity had on aia's bridge.
i also vaguely remember a picture/graph that showed the decrease of strength with an increase of humidity, but i can't find it.