Elevated Bridge B/C

[nejanimb]

Holy cow. 1st did blow everyone away... 3615 is absolutely ridiculous. Just for curiosity's sake, do you remember how they did it?

[Balsa Man]

History lesson from 2006 Bridges at Nationals, top 6:

Thanks for providing; always wondered what the actual Nats #s were. At the time there was no posting laying them out. Confirms my understanding of where Peter's bridge would have placed, had they made it to Nats. I'd also be interested in any comments on how the winner got to such an awesome efficiency. It was so nice this year to have the the full picture- at least near the top- thanks again for that, too.

The problem with building "L" shaped sections as structural chords is that they tend to buckle perpendicular to a plane at 45 deg thru the faces of the "L" when loading in compression,

Absolutely - and its true - at least that's what we've seen in testing - for both "L" shaped sections/members (i.e., angle irons w/o any core), and square cross section pieces with angle iron lamination. Just for clarification, I'm not suggesting, and we haven't tried using "L" section pieces as compression chords, just "L"/"angle iron" lamination on a square core. The up-side of the preferential failure plane that is you know the direction/plane you need to have bracing in to control column buckling. That takes us, of course out of the relatively simple matter of individual pieces pieces, forces, and failures into the overall structure dynamics - discussions for another day. As rjm says, "the best advantage of a particular chord shape depends on the structure as a whole."

As tension pieces, "L" sections offer no special advantage other than sometimes the edges make convenient gluing surfaces.

Again, absolutely. Frankly, never even condidered - or seen -such an approach. For chords in tension, the use of paired "tension strips" as I've discussed before (and seen in the photos I've posted to the Gallery), has worked very well for us - 1/64th thickness, relatively high density- density and width depending on design load.

[nejanimb]

Balsa man, do you just mean the scores from the top 6 this year? Or have you actually found images of what they did?

Also, where do you find balsa down to 1/64th thickness? I can't really even find 1/32 thickness wood anywhere... which supplier do you use? The thinnest wood we used this year was 1/16th, but I definitely think there were instances where thinner wood would have helped.

[robotman]

where do you get your wood from?
when i did towers we used an online supplier and got very nice and consistently strong pieces and you could get any size you needed the only drawback was it is a bit expensive we only uses that wood we ordered for 1 test tower and than our competition

[Balsa Man]

Balsa man, do you just mean the scores from the top 6 this year? Or have you actually found images of what they did?

Scores- with weights and weight held - # provide a "picture" of how it went...

Also, where do you find balsa down to 1/64th thickness? I can't really even find 1/32 thickness wood anywhere... which supplier do you use? The thinnest wood we used this year was 1/16th, but I definitely think there were instances where thinner wood would have helped.

Specialized Balsa in Loveland, CO - check out their website; they do lots of mail order business; Jake, the owner is great to work with - and its really great having him just a few miles down the road. In addition to weight-graded sticks, they have sheets, down to 1/64th, which you can also get in weighed/weight-graded.

[jander14indoor]

As usual, you have a cost/time tradeoff. With money, you can order almost any quality & size of wood you desire/need. With time you can find any quality and then sand to any thickness you desire/need.

The first isn't too expensive if your demands aren't too high. But costs go up exponentially for linear increases in capability. Nothing unique to EB, tends to be a truism in ALL engineering.

The second replaces money with your time, in about the same ratios.

If you choose to sand your own, several things to watch
- Start with good wood, why waste time on bad stuff!
- Use FRESH sandpaper
- Don't force it or press too hard, this compresses and distorts the wood
- Sand evenly

Jeff Anderson
Livonia, MI

[blue cobra]

Random Question:

Short of building (and annually changing) a hopper to test with, is there any way to test a bridge at home that is better than other methods? During the season I can use the school's testing apparatus, but if there is a way to get a very good idea at home without spending much money, I would appreciate if someone told me.

Now to contradict myself, is there a place where I can get a diagram or a description of the loading block (is it a 5x5x2 hunk of anything with a hole in the middle for a threaded rod)? We have quite a bit of materials around our house, and I may actually be able to make a testing device.

[Balsa Man]

Short of building (and annually changing) a hopper to test with, is there any way to test a bridge at home that is better than other methods? During the season I can use the school's testing apparatus, but if there is a way to get a very good idea at home without spending much money, I would appreciate if someone told me.

Go back to the Gallery 1048|39/P3240147Safety_Tower.jpg What the safety tower is sitting on is a base to test a bridge, or a tower. Its 7 years old, been used for both. The lumber cost a few bucks. You just need to get two level surfaces - and shim till you get it level (we usually use a corner of the kitchen counters). To load it, all you have to do is add weight to.....something hanging from the load block. We've used both a 5 gal water jug-adding water, and a bucket, adding sand or gravel. Be nice to have hopper rig, sure, but not needed. Notice that while the base board is plywood, the blocks the legs are sitting on have a piece of Plexiglass on them, because of the "smooth, hard surface" spec for testing surface. Building what you minimally need to test is easy - duplicating a hopper rig would be harder and much more expensive.

Loading block? Again, look at the photo; this one happens to be out of a piece of Rosewood scrap- any reasonably hard wood will work; yes 5cmx5cmx2cm, worth being precise when you cut & drill it

[sewforlife]

This was the company you were talking about, right Balsa Man: http://www.specializedbalsa.com/ ?

[sewforlife]

Also, a question, since they have already weighed and machine sorted "pre-set" balsa, wouldn't the weight change whilst being shipped throughout the country?