Elevated Bridge B/C

[rjm]

It is no accident that the precision of the numbers in EB are given to the tenth of a centimeter. Clearly stating the precision of measured quantities also clearly defines the limits of error or variance, and should give everyone a common basis for knowing what's allowed and what isn't. Significant figures are a legitimate part of measurement. We have no absolutes.

Whether infinite precision for a stated maximum or minimum dimension is applied will depend on the person running the event. A judge who chooses to allow no positive variance can rationalize that. To my mind, it is just another measured quantity and subject to errors and limitation of precision, like any other measurement. However, as Fleet says, why take chances? Your measuring stick and how you read it, and my measuring stick and how I read it, may be a bit different, maybe even enough to DQ your bridge. So, don't cut it so close that you are counting on getting credit for that last half millimeter. In differences over measurement, the judge is going to prevail.

BTW, as I read it, if the rules said 15 cm instead of 15.0 cm, that doesn't mean infinite precision for 15 cm, that's only two significant figures.

Bob Monetza
Grand Haven, MI

[jander14indoor]

Since my name was mentioned on this sig figures stuff, thought I'd pipe in.

First, NO ONE measured .004 cm with a ruler. It would be a challenge to do that with good machinist caliper or micrometer. Something is wrong somewhere in that story.

Second, sig digits, I suspect SO expects normal scientific & engineering practices to be used, so no special clarification needed. Note, this means that for requirements in tenths, the JUDGE has to be able to know his accuracy to hundredths, an order of magnitude higher than the requirement. Rationale for that in a moment.

Third, I know we design most of these technology events to be judged quickly and efficiently using gages whenever possible. The intent for bridge is to use a carefully measured and built block and slide it under the bridge or for Wright Stuff, to have a carefully measured gap and the wing max span fits within. And when designing those gages, this sig figure issue must be considered. Note, while often used, a meter stick or ruler is a CRUMMY measurement device. As noted, it might not be straight, might not measure the correct point, etc. But be nice, event supervisors are almost always unpaid volunteers. Give them a break, or become one yourself and do better.

So, how do you do this correctly.
Lets take one of those min measurements. Rule says bridge must clear say a 10.0 cm object. A properly built gage should be 9.95 cm tall to make sure that 10.0 plus minus 0.05 object passes. Note, this is where that extra order of magnitude comes in over the rules. If you built that gage to 10.0 cm, it could be anywhere from 9.95 to 10.05 cm. At one end of that range everything is OK. At the other, you end up second tiering some properly legal bridges! And you wouldn't know which you have! So rules to tenths require judges to work in hundredths and so on.
Now for a max measurement. Say 15.0 cm max height. Here the gage should have a bar set at 15.05 cm above the reference surface, same logic as above.
Note, I introduced the concept of reference surface. Properly this should be a certified flat granite plate or similar, not generally available. Just hope the table top the judge uses isn't too warped.

Now, once you have this gage, judging is simple and unambiguous. Devices either fit or they don't. It might not fit by a VERY small amount, too bad. You've already been given all the room allowed! If your device touches my gage, you lose.

So, what do you as a competitor do? Like others have said, the consequences of being second tiered generally far outweigh the theoretical benefit of that extra fraction of a millimeter! Example, Wright Stuff rules say 40.0 cm max wing. The difference in performance between a 40.0 cm and a 39.9 cm wing is 1 part in 400, 0.25 % less than a half second on a three minute flight! I'm good at Wright Stuff and I can't fly from flight to flight consistently enough to demonstrate that performance difference! Analyze the problem, determine the benefit, and stay inside the rules by your comfort margin!

I suggest one of you bridge experts throw some variance like that into your bridge programs. What's it worth to lose 1 mm on clearance and 1 mm on max height? What percentages do the loads change? That will tell you how much to push it. Maybe .5 mm is worth it.

But under no circumstances should you build exactly to the limits as you measure it. You are just asking to be second tiered. Poor strategy.

Finally on measurement. Scales are dirt cheap, less than $20 for 0.01 gm precision. You can make your own to 0.001 gm for even less. Length is harder. A good steel ruler is OK to about 0.5 mm, and not much money. Forget wood. A 30 cm vernier caliper to 0.02 mm is less than $20. This is science, precision tools are a must, but nowadays, high prices aren't. Oh, and learning to properly use measurement devices is one of the objectives.

My thoughts, no official SO position from here, not my perogative.

Jeff Anderson
Livonia, MI

[boomtech]

Indeed, it wasn't a wise decision on my part to cut measurements so close... thanks for the input, guys. At least I'll be able to account for it soon, at nationals.

[computernerd4826]

at state we got caught up in the action, and added 2 braces on the top of the bridge 2 nights before and didn't check height
(this bridge was close to 15 cm) and it ended up being 2 mm too high at state and we got 2nd tiered

[fli754]

Not sure if this was said or not, but is an arch design for a bridge better or worse than a uhh.. not arch design?

[nejanimb]

My personal opinion is that arches don't work. I'm sure someone has come up with a decent design that works with an arch, but I've personally never seen on that is successful.

[fli754]

Alright, just wondering. I've tried testing arches and they seem to do poorly compared to the not-arches bridges

[andrewwski]

An arch design in theory, on paper, is excellent. But as far as building it goes, not so much. It's extremely difficult and time consuming to build a successful arch from balsa or other types of suitable wood, and you'd be much better spending time building more non-arch bridges.

I saw a lot of arch bridges this year, but none of them had any success.

[croman74]

I believe that Aia and some other users had tried out arch designs, yet they all said that they didn't do as well. I think that you can come up with a perfectly good design without using an arch.

[Aia]

My high score with an arch bridge was about 1200. However, I did not see the potential for the design to get much higher, so I stopped testing arched bridges.

The major problem with arches is that you have to weaken the wood to get the curve. In addition, curves are hard to get symmetric. Any balsa stick is inherently different from another, so the curves vary a little from one side of the bridge to another.

I'd recommend just working with straight pieces and save yourself the trouble. If you worked really hard, you could probably hit 1300 or so with an arch, but in my opinion, you're better off putting your time in elsewhere.