Elevated Bridge B/C

[rjm]

Yes, that low. Scores seem to be lower generally than I'd expected. Raw scores are hard to come by, but the top six in MI Div C were 1572, 1419, 1367, 1240, 1005, and 908. The top six in MI Div B were 1376, 1357, 1266, 1265, 1158, and 1109. Our regional scores were a little low also. I'd expected that building to the new dimensions would be offset by the experience of building in the second year of the event, so the scores would be very similar.

Bob Monetza
Grand Haven, MI

[packer-backer91]

For reference, Michigan State Tournament:
Div B top score: 1376
Div C top score: 1572

Jeff Anderson
Livonia, MI

And no, I don't have pictures, sorry, I was at headquarters all day.

THAT LOW?!?

What happened to Troy's 2000? I'll go ask them I guess.

I managed a 663. Not bad, I guess.

you can blame it on the weather too. My schools bridge was pre-tested and got 1540 [I think, 11madmic said it before] then their bridge only managed 1240. My teammate also said he watched Troy's bridge brake and said that the student was very angry with the performance of the bridge I bet that the design if it would have held as much weight as planned would have been back in the 2000's again this year. For example my mouse Trap vehicle weighs exactly 1 pound but our time when I ran it on Saturday would be much more than that one pound [1.115 based on how much slower it became], the humidity killed the efficiency of the bridges. I think I herd a similar complaint at Nationals in Georgia last year.

[Littleboy]

My bridge was pretested to have an efficency between 1150-1200. But the humidity killed everyone.
I think all the humidity might have weakened to wood besides adding weight.

[Balsa Man]

I only have a little bit of data to base it on, but that data suggests that high relative humidity is not a significant factor in balsa (or bass) strength. A few years ago doing some compression testing, made up a set of 1/8th light balsa with angle iron lamination. Tried to make the same, but with both wood variability and lamination variability, I'm sure there were differences. Tested some pieces one evening, and some the next. First evening was dry, rain came in the next. Saw about 15% variation, with the lowest actually on the first(dry) day. Don't have any data on tension at high and low humidity, and don't have any bass data at different humidity. The amount of variation is within the range seen across the board (~20%). High humidity does increase mass (most in light balsa, least in bass), but I'm inclined to think its not a big factor in strength; certainly not enough to account for, say a difference in efficiency from 1500 to 1200. I could be wrong.

I could also be wrong in suspecting something else on the reports of "it was pre-tested to X efficiency and only made Y, but I have to wonder. If a bridge is.....close to the edge- i.e. designed to just carry full load, and its taken in testing very close to failure, there is a significant chance of stressing something enough to reduce its strength- breaking some wood fibers/introducing some glue cracking/peeling away, so that the load it can carry the next time is reduced. The closer the test load is to maximum capacity, the greater the chance of some damage.

As to scores this year and last, I figured at the beginning of the season it was going to be close. Folk coming up the learning curve from last year would work to increase efficiencies; the new specs, though, worked in the opposite direction, necessitating some additional wood, including longer legs. With the inverse square relationship in column strength vs length, a 25-30% length increase in legs would mean not just a bit more wood, but either stronger wood, or more bracing. The numbers coming in suggest the balance between these two factors is working out pretty close.

[matthew8817]

Well when it comes to C- division, I cant wait to try to give advice to the nebraska team. I'm only B- Division but in my city there are two high schools (east and west) that compete with each other. I tried to give West advice and they wouldnt listen and ended up losing to East. Now to see if the East bridge builder will take advice and maybe get above 300 efficiency..... Nebraska isnt exactly a first place bridge-building state. O_o

[StampingKid]

I am going to concur that humidity alone should not be a major factor in underperformance. We tested in near 100% humidity during a rainstorm inside a buidling with no windows and no air conditioning per university energy conservation policy and hundreds of human heat factories and while we picked up .6 grams the difference in performance was less than 10 percent related to added weight gain. The dynamic I think that has been at play here is that the height and other dimensions are adding multiple failure points not in play last year. First, there is leg failure vis-a-vis buckling with the extra five cm of height which was not present last year. Second, last year the main horizontal tension member could easily be ran leg to leg with diagonal tension members on each end from the base to apex but this year a lot of designs had a near elliptical football shape of the truss frame with separate diagonals from the base and the apex which emphasized good building and gluing in that critical joint. And third, the compression members at the top needed to be real good wood as they had greater forces as the longer clearance block expanded their length and angle. I consider my failure at 13.9 kg and 1545 efficiency a success when it failed in the wood in those upper compression members as might be expected as it meant I satisfactorily solved all of the other. If you ran the numbers on your design and it failed at the point of the highest forces and all other things being equal, humidity might have been in play. But all other things being equal should be read that the design had been tested to failure with consistent results. As has been pointed out on the wright stuff forum, a three minute plane is a plane that has actually flown three minutes. A 2000 bridge is likewise a bridge that has actually held to 2000. Congrats to all the winners as regardless of the efficiency you were the best at figuring out the problem---unlike baseball homerun records during the steroid era, i doubt that there will ever be an asterisk on a science olympiad record for humidity.

[matthew8817]

The dynamic I think that has been at play here is that the height and other dimensions are adding multiple failure points not in play last year. First, there is leg failure vis-a-vis buckling with the extra five cm of height which was not present last year. Second, last year the main horizontal tension member could easily be ran leg to leg with diagonal tension members on each end from the base to apex but this year a lot of designs had a near elliptical football shape of the truss frame with separate diagonals from the base and the apex which emphasized good building and gluing in that critical joint.

I have seen a lot of this new design. I remember last year being so happy that i could have a peice going diagonal from the center of the cross peice to the legs in the bearing zone. If i could still do that i think my bradge could be much better. But whatever. Rules are rules I supposed.

[Balsa Man]

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

Very good analysis
Agree.

[Littleboy]

It would be interesting for someone to make a last years bridge and see what they get with it being the end of the second year.

[blue cobra]

It would be interesting for someone to make a last years bridge and see what they get with it being the end of the second year.

I may try that over the summer. I'd bet I could break 1500, maybe more.

Also, I just e-mailed pictures of my bridges from this season to DS, so they should be up soon! I had no good pictures from before testing though, but none of them were destroyed.