Elevated Bridge B/C

[nejanimb]

Thanks BalsaMan! I saw Poudre got something back in the 50s... Are you privvy to what happened? I would guess that's a Tier 2...

[Balsa Man]

Thanks BalsaMan! I saw Poudre got something back in the 50s... Are you privvy to what happened? I would guess that's a Tier 2...

Uh, yeah, that's a big 10-4 on the Tier 2 for Poudre. I was really sorry to hear about it. They are our big cross-town rival, but once State's done, we're always glad to help them for nats. We made our jigs/tools available, and broken regional bridge to see construction detail. Went through wood specs for each of the pieces, passed on some wood left over. We also passed on the height gauge, span gauge, and clearance block I'd made for Regional & State Bridge judges to use - which I know were accurate. From e-mail with their coach, and a very brief phone discussion with their builder:
After building, a) when they got there, it had a significant weight gain (fm 12.4 to 13....1 or 2) b)they checked height with a ''cheap" ruler, read it as too tall, sanded a fair amount off the top, and still measured 20.05 at check in, and weren't allowed to withdraw/sand to fix-Tier 2. The top sanding apparently thinned the top enough to cause it to fail at around 8.7kg. What I don't know is why they didn't do leg end sanding (as opposed to top side) - unless they put the main tension members on a bit low, and didn't have room. The MTMs are the only piece the jig doesn't ....."lock-in"; there are only marks for alignment. One of the tricks on this design I'd alluded to, but hadn't spelled out, is putting the MTMs on pre-stressed. On an aluminum bar with little ears, both strips get put on one end, then you pull 11kg of tension (which stretches them about 1.3mm), and glue the other end down. They twang like guitar strings. That bar with the MTMs on it slides onto the underside, and you glue where it crosses the "main joint." When you cut the MTMs outside the ends of the bar, it pulls the sides in, putting in the unloaded deformation I'd discussed earlier. A definate possibility they didn't push it far enough up when building.

I was very happy about how the girls from Preston (Jr High from Fort Collins) did in B -13th; their best event. It was a B-configuration of our last year's bridge; same basic tooling approach.

[nejanimb]

Gotcha. Shame about Poudre - very unfortunate. We were extra, extra conservative on staying within specs; at least a couple millimeters at all sides. Are you still thinking you'll put up pictures of your design? It's cool that the image gallery is now populating itself with more photos with everyone being done.

[Balsa Man]

Gotcha. Shame about Poudre - very unfortunate. We were extra, extra conservative on staying within specs; at least a couple millimeters at all sides. Are you still thinking you'll put up pictures of your design? It's cool that the image gallery is now populating itself with more photos with everyone being done.

Yes on the pics. I'm going to ask my (more digitally proficient than I) son to do that this weekend. The gallery is becoming both a great resource, and a neat history - do want to have our part in it.

We ended up a little less conservative on clearances than intended when the jig was built; <1mm on height, about 2/3 mm on block clearance, and about 1.5mm on span; didn't appreciate the amount of structure bowing the pre-tensioned MTMs and leg TMs put in; kinda humped it up....

[smartkid222]

Towers next year for sure, probably new scoring equation, still efficiency, more emphasis on weight held.

New score equals weight held squared over mass of bridge. Still max 15 kilo load. Goal is to motivate students to get closer to 15 kilo capacity.

Jeff Anderson
Livonia, MI

woahhh. so that means that the #1 Div C bridge:
1. 6.19, 14725, 2379
under that equation would have an efficiency of 35,028,372
?!?

[eta150]

Exactly

[nejanimb]

We would've gotten second with that scoring .

Interesting concept though. I kind of like the incentive to go higher on weight held... although that also leaves a lot more room for problems if your structure collapses early. I guess that's a good thing - encourages you to be safe and use sound engineering practices - real structures ought to be designed to never fail! I definitely wouldn't like going back to where you'd get second-tiered if you didn't hold full load though. Maybe there could be a minimum load for tier one that was different than the max load? I sort of hope they wouldn't call that new version "efficiency" though - I like how, with the current name, efficiency actually is what it is: a bridge that scored 2000 actually supported two thousand times its own mass.

[Littleboy]

I will finally break 1500 efficency. It would be easy for most of us to get well over 10 mil

[lllazar]

Man that sounds pretty cool...ive always built my prototype design and tested it on as much sand as it can hold, and then worked down/up on the mass of the actual bridge...

[rjm]

Interesting twist on the scoring for Towers. I was unable to attend the meetings on Sunday to hear the rationale or comment on the extent to which this idea will weight the scoring. Hopefully, whoever came up with this one thought it through and tested some scenarios. Up through the 2001 season, structure events were scored as structural efficiency + 10% of load held. The 10% was a bonus to encourage competitors to hold more weight, but it was not so overwhelming that it discouraged teams from taking risks with lighter structures. For a given bridge mass, the scoring increased linearly with greater mass held; across a range of bridge masses, it was roughly a hyberbola. That system encouraged higher masses held while still allowing lighter bridges with lower masses held a decent chance of winning.

Scoring by using the mass held squared is not analogous to the structural design of beams, cantilevers, or bridges loaded with concentrated loads. Strength formulas for beams or bridges resisting concentrated loads use the load as a direct multiple; formulas using distributed loads use the square of the distributed load, but that's not in play here. Basic column buckling formulas such as Euler's formula use the first power of the load to predict a buckling threshold as well, and while other formulas such as Johnson's parabola or the secant formula are not first-order, they are not similar to using the square of the load either.

Aside from an arbitrary weighting scheme for scoring, was this formula intended to teach students anything about structural design or efficiency? I suspect it will simply squeeze out all of the low-load structures. It reminds me of the days when breaking at less than 15 Kg was second tiered.

Bob Monetza
Grand Haven, MI