Elevated Bridge B/C

[sewforlife]

Was this the picture of your triple load bridge: http://gallery.scioly.org/details.php?image_id=1141 ?

that was someone's triple load bridge
balsa man has his own subgallery http://gallery.scioly.org/categories.php?cat_id=39

sorry again. I meant nejanimb

[nejanimb]

Yes, that was my bridge.

The one Balsa Man said his son made was in the low to mid 2000s, if I remember correctly, which would have been a medal at nationals that year.

[sewforlife]

And your bridge achieved 1300 efficiency (it's very nice by the way).

[nejanimb]

Indeed. And thank you! Although I'm not entirely happy, looking back, with the design of it, I'm very proud of the construction.

[Balsa Man]

I'm curious though - I know Balsa Man had said they had a really good bridge that year. Balsa Man, would you mind describing and/or posting pictures of that year's design? I know I'm at least very curious.

I'd be more than glad to, but it'll be this weekend before I can do it - really busy at work these days. I'll put a few photos in the Gallery, and post here when they're up.

It weighed 5.8 gr, broke at 14.8-something kg; about 2,550 efficiency (2nd place at State that year was, as I recall, less than 1,500 - it was a 6.2gr bridge that failed around 9kg). From reports that year, we do believe it would have medaled at Nationals. Damage from failure was surprisingly minimal. Load was at the C-position (at one end). The design is.....interesting - hard to describe w/o a lot of words. Its a modified Warren truss - with the mods coming from the significantly different loads on members at center and end loading positions. It was the first time we used a couple of key things seen in our bridge this year - paired 1/64th tension strips, and angle-iron lamination on compression members. Pictures will make things clear.

[nejanimb]

Thanks, Balsa Man. I'm looking forward to seeing the pictures. I've always wondered what might've been a way to improve on the general idea of my bridge from that year.

One other thing that you mentioned but still actually applies to other structure building is iron-angle lamination. I know we discussed it earlier in this thread, but I'm still not entirely clear on how it works or what the point is. Can anyone give a full explanation, perhaps with a diagram and picture?

[Balsa Man]

One other thing that you mentioned but still actually applies to other structure building is iron-angle lamination. I know we discussed it earlier in this thread, but I'm still not entirely clear on how it works or what the point is. Can anyone give a full explanation, perhaps with a diagram and picture?

Well, I tried to do a diagram using dashes.... that doesn't work.

Description/visualization-
Imagine a square- looking at the end of a member. Let's say 1/8th square. Along the top of it, glue a 1/8th wide strip. Then along the right side, glue another 1/8th wide strip, so that the top edge of it is flush with the right edge of the strip on top. That will form an "angle iron" along the top and right sides of the square cross-section piece.

The core - the square in the description above is done in low density. The laminations are 1/64th thick, relatively high density, w/ density depending on how strong (stiff) you need the member to be.
This approach is used for stiffening a member in compression, to prevent column/buckling failure.
If you take the low density core, unless its very short, it is quite flexible; it will fail in a buckling mode under pretty low load. If you take one of the lam strips, it is very flexible. If you put two of them together at 90 degrees - making an angle iron, it becomes quite stiff. If you glue that angle iron to the light core, the resultant member becomes very stiff. You can also get more stiffness than a low density piece by going to higher density - without lamination, but what we have found is you can get a given amount of increase in stiffness at a lighter total weight by doing angle iron lamination..... That's it in a nutshell.

[StampingKid]

BalsaMan, What type glue are you using to laminate the angle iron to the core and is it spot glued or a true lamination along the entire length?

[Balsa Man]

BalsaMan, What type glue are you using to laminate the angle iron to the core and is it spot glued or a true lamination along the entire length?

Slow or medium CA depending on length you're working with; true lamination- thin, even glue layer, making sure the edge of one lam is fully glued to the side of the other.

[rjm]

History lesson from 2006 Bridges at Nationals, top 6:

1. 3.88 g / 14.025 kg / 3615
2. 6.87 g / 15.000 kg / 2183
3. 6.61 g / 14.415 kg / 2181
4. 6.69 g / 14.055 kg / 2101
5. 7.62 g / 15.000 kg / 1969
6. 6.76 g / 13.050 kg / 1930

There was no contest for first place.

Load point was at the center of the bridges.

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, and if the legs of the "L" are deep, the edges can roll and get wavy. We tried "L" sections for tower legs and found that they worked if you supported the outer edges of the legs with the bracing between legs. As tension pieces, "L" sections offer no special advantage other than sometimes the edges make convenient gluing surfaces. As with any structure, the best advantage of a particular chord shape depends on the structure as a whole.

Bob Monetza
Grand Haven, MI