Elevated Bridge B/C

[Aia]

Via Soinc store:
Manuals, CDs, DVDs, etc. can still be ordered but, 2010 revisions will NOT be available until Sept. 10.

So factor in slow shipping, maybe a month from now?

At best, probably mid September? Not only that, I think rjm alluded that changes would be made to the draft rules for elevated bridge. So, even if you've seen the draft rules and have a sense for new specifications, bridge will likely have new guidelines yet.

[nejanimb]

I just hope the new rules don't totally obsolete the designs I've been working on.... I'd understand if they didn't fit spec, but I at least hope the principles will still apply.

[nejanimb]

Question: Where do people get wood as tiny as 1/32 and 1/64? I'm partial to specializedbalsa.com, but the smallest they offer is 1/32 by 1/8. Does anyone know of a different source from which to get those really tiny pieces?

[smartkid222]

Question: Where do people get wood as tiny as 1/32 and 1/64? I'm partial to specializedbalsa.com, but the smallest they offer is 1/32 by 1/8. Does anyone know of a different source from which to get those really tiny pieces?

sometimes finding wood like that is hard. most hobby shops i know i dont' carry it. the one i go though has 1/32 sheets. Idk what "i'm partial to specializedbalsa.com" means. but they have 1/32 and 1/64 SHEETS, dont look under strips. You are gonna have to strip that to the sizes you need. also other online places probably have it too. maybe hood's woods. idr the other ones that people use.
http://www.specializedbalsa.com/
i cant' find the link for hood's woods.

my question is: are you really planning on using wood as tiny as 1/32 and 1/64 on your bridges?

[fmtiger124]

Question: Where do people get wood as tiny as 1/32 and 1/64? I'm partial to specializedbalsa.com, but the smallest they offer is 1/32 by 1/8. Does anyone know of a different source from which to get those really tiny pieces?

Idk what "i'm partial to specializedbalsa.com" means.

I think he means thats the site he typiclly looks for wood on so he might not have looked so hard for the other kinds of wood

[nejanimb]

By "partial to it," I mean "tend to favor it." I'm not particularly interested in stripping my own pieces, since I have no idea how, but perhaps I'll try it if I can't find an alternative solution. Really, what I'm looking for is somewhere I can find that sells a full variety of sticks down to 1/32 and 1/64.

I definitely can think of things I'd do with those dimensions. Of particular interest to me would be 1/4 by 1/32 (which, fortunately, specializedbalsa.com sells) and 1/32 by 1/16, and perhaps even 1/64 by 1/8. If I can't find all these odd dimensions, the couple 1/32 width sticks that specializedbalsa does sell I will almost definitely use, but if someone has another good wood source online that'd be good to know!

[andrewwski]

My local hobby shop sells 1/32" square...unfortunately hobby shop wood can be inconsistent and such a small size is very difficult to judge. I cannot think of a scenario where you would want to go to 1/64" other than gussets.

[nejanimb]

Definitely useful for gussets, but I have another theory that I'd like to test with using thinner wood.

From what I can tell, although compressive strength is definitely dependent on cross-sectional area, it seems to me that tensile strength is independent of cross-sectional area. Also, I think that denser wood is better than less dense wood because it is less likely to have irregularities. Thus, my theory is that, for a member in tension, a piece with a smaller cross-sectional area is better than one of equal mass with a larger X-sec area, since it should have the same strength while being less likely to have irregularities. For example, one of the main tension members in a current design is 1/4 by 1/16 - if I could replace this with paired 1/32 by 1/8 strips, the pieces can be twice as dense at the same mass, and I think this would be superior.

Thoughts on this notion?

[JimY]

Nejaminb, yes, you're right on the money, in my eyes.

I'm hoping that you and others will consider taking this too the extreme too, by using bass instead of balsa for the event! Except for maybe a few minor pieces, I coach bass construction. The pieces are definitely smaller and can be tough to work with, but the results speak for themselves: 4 wins at nats (2000, 2001, 2003, 2009) plus a 4th place (2006, though this one was almost all balsa). Maybe it was just the new event last season, but if you saw the B division results that RJM posted after nats last year, the first place entry beat #2 by close to 20% in the final score, which is pretty much a landslide. All I read about in this thread is balsa, balsa, balsa. I say wrong, wrong, wrong!

My main basswood source is northeasternscalelumber.com. Gussets in last year's design, for example, were 3/32" wide x 0.020" thick x 5/8" long. This was a tiny amount of mass that never failed!

I would consider using more balsa only if the calculated forces were low enough to do so, like in 2006. This was not the case last season with the compressive and tensile stresses significantly higher than on any of our national medal entries from the past. So, this just cemented us to bass, pun intended.

So, wait for the rules to come out, calculate out a few different designs, see what the stresses are, then choose your materials. You can actually start this by taking the node points I gave on last season's B division nationals winner and designing the materials for each piece such that the total mass of the wood used to interconnect them (with 7 interconnecting triangles) is about 2/3 of the total mass. I would like to tell the group what percent of this mass was used for the main compression pieces, but that would be giving away too much information, much like aia not including their 7th place C division photo in their gallery.

[nejanimb]

Haha.

I have never actually tried working with Basswood. Considering I have the full season ahead, perhaps I'll give it a try - it'd at least be interesting to try it and see how it goes. Besides the track record to back it up, what about bass do you think makes it a superior choice of material?

That site you mentioned looks really great - thank you! They offer such a wide variety of shapes, which is exactly what I've been looking for. Do they allow you to specify a target mass-per-stick that you want, as specializedbalsa.com does? Have you ever gotten this specifically?

One question I have about that design sketch you described (since I actually took the time to draw it out): why the inverted triangle directly under the load point? Every other part about the design I understand, and it makes a lot of sense to me (you get to rely a lot on the tensions), but that one part baffles me. It seems like it would force the one straight horizontal tension to try to support a vertical load, which would result in a net force downward right in the center, which is never what you want (you want net force at every node to be zero except under the loading block, which should support 7.5kg straight up per side, and at the feet, which should be 3.75 straight down per foot - correct?). Perhaps there's something I'm missing. I understand if that's a question you don't feel comfortable answering, but I've been curious about that.

As far as how much mass should be in the main compressions - for us it was more than 65% in last year's design. I'm not sure if we did it right necessarily, but far and away that was the heaviest part of the bridge. I think a lot of newer builders don't put proportionally enough mass into those compression members (or, rather, put too much into tension members and auxiliary pieces). I think we'll fiddle around with exactly how much goes where for this year, but it's definitely a lot.