Elevated Bridge B/C

[croman74]

If you just design your bridge to be 14.5 cm high, you know you don't have to worry about the height, and the score won't be too affected.

[croman74]

I just tested my bridge. It had a score of 478. It broke at the joints. I believe that the first break was at an end joint, so I'll just use a gusset.

[andrewwski]

Wasn't in this event, but chiming in...
Isn't there a reason the rules only have 2/3 significant digits (to .0 cm) in the rules? I'd think that the accuracy of measurement would be the same as that of the rules. Of course you should attempt to play it safe, but to me, that bridge would be legal.

Good point. I actually agree with you in this case. If it's specifying it to the nearest .1 cm, that's the way it should be measured. Why do I think this? Because it's implying the measuring equipment only needs to measure to that accuracy. Say a team only has a measuring device that measures to the nearest .1, yet they use one at competition that measures to .01...there could be problems.

So I'm changing my mind - the bridge shouldn't have been second tiered IF it was .004 cm over the 15.0 cm maximum. However, I still don't believe it could be measured to that accuracy.

However, if it's .05 cm or more off, then it definitely should be second tiered.

I really do think that S.O. needs to put a general clarification into effect on significant figures. If they mean infinite precision, that's fine, but they should say so.

[smartkid222]

that's what i was basicly saying in the last part of my last post.

In the Wright Stuff rules it says that the motors have to 1.5 grams or less. Now the scales at the competition i procotred were purchesed by the Event supervisor ($200 each) and it was accurate to .01g. so what do we do with if the motors were 1.51, 1.52, 1.53, or 1.54? In my opinion i would allow them. the rules are only accurate to 0.1g. Also my MS doesn't even even have a scale to that accuracy and it was a regional competition so if schools can't affor expensive equipment they should be penalized for that. But really a .01 scale can be found for $12 online and mr. anderson recomends making your own. At the competition i proctored this case only came up with one motor and the ES said that it should be cut down to 1.50. I also agree that a clarification would be good.

[Balsa Man]

Interesting discussion on meeting measurement specs.
I agree with andrewwski's take- if the dimension is stated to 0.1, then the implied precision is 0.05, but there should, indeed, be clear general criteria on the precision of measurement to be used.

If you just design your bridge to be 14.5 cm high, you know you don't have to worry about the height, and the score won't be too affected.

Yes, but not entirely. The height of the truss portion - the distance between the top and bottom members; 7.5 cm for C, 5 cm for B has a.....non-trivial affect on the load on the "up and down" pieces in the truss- the pieces joining the top and bottom members. As that height goes down, forces (everything else being equal) go up. You can check it out in the jhu bridge builder app. So, half a centimeter, you're giving something away needlessly; not a lot, but something. We make sure we're on the "good side" of any measurement spec by 1/2mm; e.g. a height of 14.95cm. This kind of precision may sound .....silly to some, but if things are "out of alignment" by 1/2mm (that much curve, or 2 pieces that are designed to be parallel out by that much), that could well be a "fatal flaw."

I just tested my bridge. It had a score of 478. It broke at the joints. I believe that the first break was at an end joint, so I'll just use a gusset.

Sorry to hear. If it was a joint break, most likely it was a joint in tension; gussets, as in one on each side, should help. Good luck on the next one. You could put together a safety tower in a couple hours; then you would know....

[croman74]

that's what i was basicly saying in the last part of my last post.

In the Wright Stuff rules it says that the motors have to 1.5 grams or less. Now the scales at the competition i procotred were purchesed by the Event supervisor ($200 each) and it was accurate to .01g. so what do we do with if the motors were 1.51, 1.52, 1.53, or 1.54? In my opinion i would allow them. the rules are only accurate to 0.1g. Also my MS doesn't even even have a scale to that accuracy and it was a regional competition so if schools can't affor expensive equipment they should be penalized for that. But really a .01 scale can be found for $12 online and mr. anderson recomends making your own. At the competition i proctored this case only came up with one motor and the ES said that it should be cut down to 1.50. I also agree that a clarification would be good.

I would agree with the ES. When someone says 1.5, you're supposed to assume that it equals 1.50000... and on and on.

I just tested my bridge. It had a score of 478. It broke at the joints. I believe that the first break was at an end joint, so I'll just use a gusset.

Sorry to hear. If it was a joint break, most likely it was a joint in tension; gussets, as in one on each side, should help. Good luck on the next one. You could put together a safety tower in a couple hours; then you would know....

I would love to construct it, but I have almost no time this week due to track meets, hockey tryouts, and an excessive amount of homework. And Saturday I have my National History Day state competition, so nothing there. I'll just make it after SO.

[sr243]

yea 0.004 shouldn't be disqualification. It could be just the ruler and not the person's fault. What i do instead cause i need to make sure i don't get second tiered is have at least .5 cm leeway from all the specifications. This kinda hurts my performance because the height is only 4cm but i'm pretty sure in WI that no one will get much higher than 1000.

[croman74]

I think that it should be a disqualification. 15 cm equals 15.000000000000000000... The last time I checked 15.00000000000000000<15.004.

[boomtech]

Blame the ruler. I'm getting another one
Although, to be honest, the difference was basically invisible. Plus, the judge measured at an angle to the bridge, which would have made the bridge height *seem* too high. The way the judge measured with a ruler is relatively significant in this case, since it is only 0.004 cm.

At any rate, I've learned. Time to build safer. At least I've got another chance at nats.

[boomtech]

I think that it should be a disqualification. 15 cm equals 15.000000000000000000... The last time I checked 15.00000000000000000<15.004.

The last time I checked the rules, they stated 15.0 cm, not 15 cm. A sig fig was included; if they had wanted to indicate infinite sig figs they would logically have specified. At least I think so.