Elevated Bridge B/C

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evbassboy13
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Re: Elevated Bridge B/C

Post by evbassboy13 » January 17th, 2010, 12:58 pm

Sewforlife, that's great! How far back was second place? What're you expecting for state?

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Re: Elevated Bridge B/C

Post by nejanimb » January 18th, 2010, 9:38 am

Sewforlife - Congrats! That's a very strong score, and a gold medal never hurt anyone! I'll also be curious to see how you did it. =).

Noda - You mentioned a few pages back that you'd be at Athens. How did it go?

Otherwise, for a heads up: Just like last year, they took video of all of the bridges at the Athens tournament, so that should be up (I'm guessing) on youtube within the week. Should be good to watch - I saw a lot of the bridges while I was there, but it's cool to see them up close on video and to catch all of the ones I missed. I didn't see Union Endicott's bridges, which were silver and bronze (I heard they were a bit over 900 efficiency, and so I'm hoping they were included in the video), so I'm very curious on that. Just from what I saw overall though, it was similar to my experience at Conestoga: a lot of bridges out of spec (though not quite as many), even from teams that are traditionally strong. Sometimes it was a case of not reading (or understanding) the rules, and sometimes it was a case of being careless and cutting it way too close. Most of the bridges I saw tended to be box trusses on legs, which meant, as other people have reported from elsewhere, that a lot of them broke right down the middle just because the span is so tough.

Congrats to all that were there!
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Re: Elevated Bridge B/C

Post by Balsa Man » January 18th, 2010, 2:54 pm

Very nice, indeed, Sewforlife!

Really looking forward to that video, najanimb. Were you seeing many just falling over (like in a lot of last year's videos) - what w/ the extra 5cm in height this year?

Been a while since I had time to get on the board......

We'll know better where we are at in a couple weeks, but it looks (at least in C Div) that 1500 is do-able.

Experimenting/testing to-date has turned us on to the value of small cross-section bass for compression members (as in smaller than 1/8th). We found more variation in density of bass than I expected, and we now have data on column/compression strength vs density. Basic approach is last year's design......stretched to meet new dimension rules. Using paired balsa strips for tension again.

All for now. Looking forward to how things unfold this year.

Len
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Re: Elevated Bridge B/C

Post by nejanimb » January 18th, 2010, 3:33 pm

Good to see you around again, Balsa Man.

I was surprised - I figured that with the extra height, the falling over problem would be highly exacerbated. However, that has not been our experience. We've been able to stubbornly continue with our method of using vertical sides with no problem, rather than having to taper them in toward the top. While there were several bridges I saw that tipped, it was definitely not the predominant failure method, and when it happened, it seemed to be because there was insufficient lateral bracing on the bridge, so the problem was with weight distribution, not balance. You'll see more when that video comes out, which I'm excited to see too. I'll have to apologize in advance, however: we specifically requested that our bridge not be filmed. =).

I'm surprised you're able to continue with your general design from last year. We built one quite similar to that outline over the summer to last year's specs (since we had never tried anything like it), and did passably well with it (972). However, I got halfway through trying to build another one like it shifted to the new specs, and scrapped my effort before even finishing it because I decided that, with the new longer clearance, it didn't make sense for me. I'll definitely be curious to see how you make it work.

[EDIT: I just spent a minute putting pictures of that bridge from the summer up in the gallery. They should get up soon - feel free to take a look, anyone.]

I would agree though - I actually think there's a reasonable chance scores will be, overall, higher this year. I don't know how hard the change was for B div, but the C division differences I think are readily adaptable, and, for us at least, the experience we have from last year far outweighs the increased "difficulty" of the specifications. 1500 is doable.
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Re: Elevated Bridge B/C

Post by sewforlife » January 18th, 2010, 5:14 pm

Oops, Sorry, I can't find an actual picture of our bridge, we just took a video. I'll get up a picture sometime this or next week when we rebuild and start to edit. we're probably expecting something over 1000 for states, just to secure the 1st place position. it was over 1000 when testing at school, but it competition the efficiency went down. we need to figure out what happened.
we don't know all the efficiencies for all the teams. all we have is our own.

did anyone else notice if div. C bridges were made of chunky wood?
all the div. C's looked at least 30 g. and they were made of 1/4 by 1/4 balsa or something else.

does anyone know if people should stain bridges (like with paint) cuz i saw one of those at regionals.
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Re: Elevated Bridge B/C

Post by evbassboy13 » January 18th, 2010, 6:23 pm

I've seen quite a few C bridges made of 1/4 x 1/4 balsa, but they always seem to be poor designs, where the builders just try to make up for strength with bigger wood. I used 1/8 x 1/8 for my first (and so far only) bridge, and did pretty well. I plan to use that size and smaller for my next bridge.

As for staining, I was just thinking about that today, and it seems like it wouldn't do a whole lot. I'm sure it'd improve the capacity a little, but my guess is that efficiency would drop pretty sharply. I've never tried it, though, so if you give it a shot (or if anyone else has), let us know. Also, I wonder if stain or paint would do anything to the glue.

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Re: Elevated Bridge B/C

Post by dragonfly » January 18th, 2010, 6:43 pm

Not all division C bridges are created equal, so there'll definitely be variation. From what I've seen this year, the bridges use mostly 'thinner' wood (thin being relative) ; while there are a few exceptions, most people have the experience of a year of building bridges and have likely learned more about appropriate wood sizes. I don't think I've ever created a structure using just 1/4 by 1/4 though... It's just SO big!

Athens this past weekend was extremely enjoyable and interesting. Some of the designs I saw were pretty awesome looking with fantastic cross-piecing and overall cool-looking ideas. Most of those, sadly, were either 2nd tiered or simply too heavy to have a quality efficiency. I also noticed that a lot of people build using solely one dimension of wood throughout their bridges, though I, much like nejanimb, am one who enjoys the diversity of shape and size in wood. I guess that's common in designs that are early in their development stages.

Six more weeks until regionals... and in that time, plenty more bridges to build.
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Re: Elevated Bridge B/C

Post by sean9keenan » January 18th, 2010, 10:29 pm

nejanimb wrote:
We've been able to stubbornly continue with our method of using vertical sides with no problem, rather than having to taper them in toward the top.
Yes, you are rather good at that stubborn bit, aren't you? :P. Are other people "tapering" in their sides?

And I agree with nejanimb, it'll be interesting seeing that bridge "updated" for the new specs, it doesn't appear as if it'll be a small change that'll do it, but who knows.

and yes... still plenty of time in the season to build some more bridges, and hopefully a few more devices for some other builds...
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Re: Elevated Bridge B/C

Post by Balsa Man » January 19th, 2010, 5:28 am

sean9keenan wrote:
nejanimb wrote:
We've been able to stubbornly continue with our method of using vertical sides with no problem, rather than having to taper them in toward the top.
Yes, you are rather good at that stubborn bit, aren't you? :P. Are other people "tapering" in their sides?

And I agree with nejanimb, it'll be interesting seeing that bridge "updated" for the new specs, it doesn't appear as if it'll be a small change that'll do it, but who knows.

and yes... still plenty of time in the season to build some more bridges, and hopefully a few more devices for some other builds...
Yes, we're running lean-in; last year at 15cm height, 3/32 in on each side; just extended that lean to the new 20cm height, so now its 1/8th in on each side.

A little more on the updated bridge. Relatively small changes, I'd say. Instead of straight top compression members, a 5cm straight/flat middle section with the ends then sloping down, oh, 10+ degrees. Inspired by looking at Aia's designs. The wider/higher clearance block means, of course, that the points between which the main, bottom tension members run are further apart (and higher above the base) than last year. Otherwise, same pieces/layout, with the legs a bit more vertical.

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Re: Elevated Bridge B/C

Post by croman74 » January 19th, 2010, 2:20 pm

So last week we tested my first bridge from this year, and let's just say it didn't go so well. One small piece on the bottom of the legs broke and the bridge tipped over. Before I had tested my first bridge, since i waited a few weeks before I could, I started making another design with a similar truss, but vertical legs, a design I feel gives it more support. It also weighs 15 grams, and I'll be testing it next week wednesday. Hopefully this next bridge will be stronger.
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