Elevated Bridge B/C

[nejanimb]

Only one side? That's kind of odd... how will they be evaluating it? I mean, if they're just seeing how light you can make it, then go light (but that's a crazy method of evaluation). If they're just looking at the kind of design you have and your skill and building technique, then it shouldn't matter how heavy you make it. And, there's no real way to test it against a load if you're only going to have one side truss...

Generally though, I agree: start heavy, and then optimize. Our method is to brainstorm, and then do a shotgun method by building and testing a bunch of our favorite ideas from brainstorming. Then, once you figure it out, build, test, and analyze the break. Then, either modify your design if it needs it, or beef up the area that broke and slim down (well, I mean just increase and decrease densities accordingly) the areas that didn't.

Speaking of which, we finished and tested our first bridge today (actually, the first bridge with the new specs - we built two new designs over the summer according to old specs)! Starting this new season off is interesting...

[fmtiger124]

Only one side? That's kind of odd... how will they be evaluating it? I mean, if they're just seeing how light you can make it, then go light (but that's a crazy method of evaluation). If they're just looking at the kind of design you have and your skill and building technique, then it shouldn't matter how heavy you make it. And, there's no real way to test it against a load if you're only going to have one side truss...

Not really sure how they are going to evaluate it, I guess weight won't matter all that much. My best guess is they are having us build to see our building skills with balsa and perhaps they will input the design into some sort of computer program

One other question: Any recommendation on balsa size? I was going to use the basic 1/8 X 1/8 but would something else be better?

[sewforlife]

Only one side? That's kind of odd... how will they be evaluating it? I mean, if they're just seeing how light you can make it, then go light (but that's a crazy method of evaluation). If they're just looking at the kind of design you have and your skill and building technique, then it shouldn't matter how heavy you make it. And, there's no real way to test it against a load if you're only going to have one side truss...

Not really sure how they are going to evaluate it, I guess weight won't matter all that much. My best guess is they are having us build to see our building skills with balsa and perhaps they will input the design into some sort of computer program

One other question: Any recommendation on balsa size? I was going to use the basic 1/8 X 1/8 but would something else be better?

I think 1/8 by 1/8 is fine to start off with, 1/4 by 1/4 is wayyy to huge for me.
I am making the new SO member build one side too. just to evaluate their joints and make sure they know what to do. They really don't, but it's mainly a test for specs and instructions.

[AlphaTauri]

One other question: Any recommendation on balsa size? I was going to use the basic 1/8 X 1/8 but would something else be better?

Yeah, I'd use 1/8 by 1/8 for most of it. If you have really high stress on a member, I might go to 3/16 by 3/16, but definitely not 1/4 by 1/4. It either adds too much weight to be efficent, or is spongy- not a good quality for a bridge.

[nejanimb]

1/4 by 1/4 is gigantic. I actually usually feel like 1/8 by 1/8 is really big too, but I know it's a commonly used size. If you want to stick to square pieces, I'd probably use 3/32 square actually. Except for 1/16 by 1/16 though, I generally much prefer non-square pieces.

FMtiger, what kind of design are you thinking about using? Have you looked through the image gallery to get any ideas?

[andrewwski]

I use mainly 3/32" square. 1/8" for high-stress pieces, but not very often.

Theoretically non-square pieces may be more efficient. I never tried them, though.

[dragonfly]

I can't even remember if I've ever used 1/4 by 1/4 sticks for an entire structure... it's just massive. Not to mention that fact that even with a relatively lightweight 1/4 by 1/4 it becomes difficult to manage and unnecessary. I've got to say though, if I was a novice builder that I would (and did, during middle school) start with 1/8 by 1/8. It's easier to maneuver to start, and poor joints and other mistakes are usually easier to spot and correctable. Then, after experience, moving on to more wacky dimensions like... what I think is 1/8 by 1/32 or something like that, everything will be a piece of cake it just takes a LOT of time, and a LOT of bridges! Good luck on your bridge fmtiger.

[RandomPerson]

Yeah I'm in exactly the same boat for tryouts as tiger.

My coach recommended a place for me to buy some balsa, is there anything I need to look for in the balsa before I buy it (like density... I dunno, I'm really not an expert here).

And does anyone have a recommendation for a design to build (that i would modify of course, I'm not looking to plagiarize), that would be impressive? I've looked through the galleries of course, but its hard to tell what would be what I'm looking for and what I'm not.

[nejanimb]

You will definitely need to consider the density. Once you get into it, you'll look at grain and stiffness, but mostly you can just feel it at the beginning. Look for wood that seems like it would hold stuff up! If you're going to be using 1/8 square pieces, I'd say wood thats about 1.3g per 36" stick is medium density, which is about what you'd need (probably).

[JimY]

Hey, did anyone notice in the event rules that they gave no maximum time to load the sand? Last season it was 10 minutes. It says MAXIMUM TIME: 10 minutes on the top of the first page, but I'm wondering what this is for. The maximum loading time is usually precisely defined in the COMPETITION section.

Also, regarding the discussion on bending pieces into arches a couple pages back, I say bad idea. The first thing that pieces in compression do before they fail is deform or curve! Why start with a curved piece that helps the failure process along. You can get away with it if you have only short lengths between interior members and cross braces between the trusses. Then the truss would resemble a segmented arch, which is actually a good idea. Note the photo on the first page of the rules. This was the B division winner at nationals from last season, which I coached. The other way you can get away with arches is to use more material for those curved members, lowering the efficiency. Remember, that the bridge in the photo had an efficiency of over 2500 and did not fail at 15 kg. Also, it was made of over 80 pieces of bass with only 10 pieces of balsa in non load-bearing locations. It had 9 structural nodes per truss. If you want more info on it, search through way back on this thread, and maybe check out the tail end of last season's thread.

As for the student that was asked to build only one truss, I'm having all three of my teams (two B and one C) do exactly the same, and its a mini-elevated truss only 13 cm long. For the middle schoolers, I'm judging their ability to do detailed work with great workmanship. I will cut the weaker students out of the event in this way, then we'll proceed in building for this season's construction parameters. Materials don't matter for the mini truss, and mass doesn't matter either. For the high schoolers, I used this as a get-reacquainted-with-bridge-building exercise and pointed out all their errors so that they don't make the same ones on their full size builds. Yea, I'm sort of a nut case in this event, but as I've posted before, the results speak for themselves. Not only have I coached 4 separate B division winners at nationals since 1999, I've also graduated two BS civil engineers over that same time span. Cool beans.