Elevated Bridge B/C

[nejanimb]

JimY, thanks for the insight. Sounds like a really cool design, and it's very impressive that you got the mass so low. It's different than many of the others I've seen that have been successful.

Is the C design very different? From what I heard from some of Valpo's builders, it sounded like it went in a totally different direction than that one was.

[blue cobra]

Balsa Man,

What are the approximate dimensions of the safety tower. I searched (by author ) and can't find it.
Also, and this goes out to anyone, what kinds of densities should I keep on hand for these bridges? I know some need to be very dense and some need to be very, er, not dense (nense?) but is that in the range of 10#-15# or will I need wood varying as greatly from 7#-20#? Also do cross members need to be very dense? FYI I'll be ordering from specialized balsa (if anyone's ordered from them, I'm having a bit of trouble). Thanks in advance!

[smartkid222]

after you build for a few years you end up with a quite large stockpile of wood. I keep them categorized by density (L,M,H). The densities i keep on hand are very varied. If i knew what i was gonna make though, i would only order around what i would need.

[Aia]

Also, and this goes out to anyone, what kinds of densities should I keep on hand for these bridges? I know some need to be very dense and some need to be very, er, not dense (nense?) but is that in the range of 10#-15# or will I need wood varying as greatly from 7#-20#? Also do cross members need to be very dense? FYI I'll be ordering from specialized balsa (if anyone's ordered from them, I'm having a bit of trouble). Thanks in advance!

I think this answer depends strongly on the kind of bridge you design. I only know the specific densities of balsa that I need after I play with the nuances of the design. I had three primary bridge designs last year, and each required a very different set of wood densities. The only constant density I can point to are the cross members. For all my designs (but again, this could be different for your bridge design), I could use low density lateral bracing without compromising any strength of the bridge.

Are you ordering from specialized balsa soon? If so, why so early?

[nejanimb]

Usually, at the beginning of the year, I like to have a wide variety of densities, since I'm never sure what my eventual design will require. While I'm experimenting with different ideas (and I, along with my team, probably tried 20 radically different design ideas this year), I pretty much ignore the specific densities of the pieces (I sort of consider it, but only very qualitatively) so I can build more - those ideas will reveal how well they work without that much attention to detail. Once we've figured out what we want, we order at least the pieces with the right dimensions, and then sort them by densities. Then, after testing to determine the right densities, we'll order specifically.

Basically, we weren't sure of the pieces and densities we wanted until as late as the end of April and into May of this year. At first, I'd suggest getting a very broad range and playing with what works, and then making it more specific as the season progresses. I agree though, that lateral bracing can be very light (ours was about 2.1 grams per cubic inch).

PS: I uploaded several pictures of this year's bridges to the image gallery.

[Dark Sabre]

The ^ aforementioned ^ pictures nejanimb uploaded:

1724|10/IMG_0436.jpg 1725|10/IMG_0442.jpg 1726|10/IMG_0462.jpg 1727|10/IMG_0518.jpg 1728|10/IMG_0566.jpg

[nejanimb]

Thanks for posting this, Dark Sabre!

[Aia]

Has anyone seen the draft rules for Elevated Bridge for the coming season?

[Balsa Man]

Sorry 'bout dropping into a black hole for the last.....few weeks; way too busy at work, but in these economic times that's a good thing I guess.

Skimming over postings, a few quick things-

I will get a picture up of the 3 load-point bridge up within the next couple weeks.

On the safety tower - Jeff, thanks for the.....appreciation of what it allows you to do. Its the closest thing to magic I've found over the years. Clarification, its not so much that it slows failure down so you can see it, but that it stops it right after it ocurrs. You can/should adjust the length of the upper chain (the slack in the chain) so the block can fall.....just enough to allow failure. With a very rigid bridge a 16th or just a tad more is enough; if its more flexible, a little more;maybe up to 1/8th. When we tested Peter's State bridge to failure, we were at about a 16th. One of the upper compression members (top cross pieces) broke - right at the edge of the load block - at the point of highest compression load, where the diagonal compression truss member from below joins the top compression member. 1/8th square stock, one end of the break displaced down - the ends of the break are actually still connected w/ wood fibers. Jeff has it exactly right - in terms of using this tool - where you would go from this point; fixing, and continuing to test the same bridge. In this case, carefully pulling the break back in line, putting "splint" pieces on - and splinting/strengthening the .....corresponding places - the other three places where the top member and compression truss pieces join. With those places strengthened, the next loading will a) more than what caused failure the first time - with that area/member/joint strengthened, and b) will be somewhere else - the next weakest place.

On the question on dimensions on the tower, it really doesn't matter; whatever will fit on the test base you use, and clear the structure you're testing, and is steady/stiff enough to hold a 15kg load without visibly flexing - just look at the photo in the gallery.

Looking forward to hearing how the new rules come out, too!

[smartkid222]

Has anyone seen the draft rules for Elevated Bridge for the coming season?

http://soinc.org/event_rotation
i don't believe there will be draft rules. just the official rules when they come out.