Elevated Bridge B/C

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sean9keenan
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Re: Elevated Bridge 2009

Post by sean9keenan » January 1st, 2009, 7:32 pm

I don't remember reading that you are allowed to disassemble the loading block... Where does it say that? I only ask because one bridge that I made did require that I disassemble the loading block in order to test. It was a simple mistake, and easily correctable (in a new design). In my next bridge I did correct it because I figured it would be a bad idea to depend on being able to deconstruct the block (or that the block would even be similar enough to work the same) at the competition without being 2nd tiered...

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Re: Elevated Bridge 2009

Post by rjm » January 1st, 2009, 9:56 pm

You SHOULD be able to disassemble the loading block and reassemble it within your bridge. Rule 6.f says, in part, "... and assemble the Loading Block, eyebolt, chain, etc., and hang the bucket..." Before you plan on doing so, you could contact the ES at your tournament to see if it will be allowed, or get a clarification on the point from S.O. No opinions expressed on this board are binding on any tournament.

Triangular cross-section tubes aren't necessarily the most stable shape. There was lots of discussion of this on triangular towers vs. square-based towers. For a condition in which the main chords are subject to buckling, like the legs of a tower or the top chords of a bridge, the bracing between chords subject to buckling work best when the planes of the structure are at right angles. I think that the bracing in the top plane of a triangular bridge tube with two compressive chords would bulge upward and/or downward. If the tube is inverted so that there is a single compressive chord (block would go inside the bridge), the compressive chord would tend to buckle sideways. It could be an interesting experiment, though.

Bob Monetza
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Re: Elevated Bridge 2009

Post by Keyreaper » January 2nd, 2009, 10:45 am

Hey I have finally gotten my balsa wood (sticks & sheets) and was wondering, if you are going to butt joints, what shape should the gusset plates be?? I was thinking square or triangle depending on angles. And if the gusset plate grain should go with or against the grain of the sticks. Thanks.

*Edit*- Almost forgot. Also, what thickness should the gusset plate be? I have tried it with 1/16th but the plate breaks easily along the grain. I just splits. I want the thickness to be strong enough so the glue joint breaks, but not the material. Thanks in advance.
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Re: Elevated Bridge 2009

Post by smartkid222 » January 2nd, 2009, 11:17 am

when you people say gussets to you refer to
guessts like this where you are adding wood to the 90degree angle in this picture
Image
or here where you are covering the whole joint???
Image


Glue failor is not good and it means that you need better glue. the material should be breaking, not the glue. I think right?
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Re: Elevated Bridge 2009

Post by Keyreaper » January 2nd, 2009, 11:28 am

Correct I do mean those kinds of gussets... preferably the second one where it is placed over the wood. And I think you are correct. The material should be breaking. Also. Have any of you considered half-lap joints???

Here's a link to an example. http://www.rabbitwoodworks.com/images/J ... -joint.jpg

I know it might seem hard (or maybe impossible) on these sizes of pieces. But, last time I checked this kind of joint doesn't really require a gusset plate. Its half butt, half lap joint, meaning it could work best as a tension or compression member, I think... Any opinions?
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Re: Elevated Bridge 2009

Post by smartkid222 » January 2nd, 2009, 2:53 pm

Keyreaper wrote: seem hard (or maybe impossible) on these sizes of pieces.
that's my opinion. too much work for me to see any really positive outcome of using it. LOL.
Im not even really sure if there actually better than lap joins, i dont' think they are.
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Re: Elevated Bridge 2009

Post by Pleiades » January 2nd, 2009, 5:59 pm

Aia wrote: Well, I'm going to be proctoring elevated bridge at a B division invitational in a couple weeks. I'll let everyone know the efficiency range afterwards. How is testing going out there? The B division building team I've been helping is struggling to hit the 15kg mark, so their efficiencies aren't competitive yet. I'm at a point where I've chosen a final design and every half a gram lost is an uphill battle. Anyone else out there have updates on their structures?
Has the invitational happened yet? if so, do you have the ranges? thanks :D

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Re: Elevated Bridge 2009

Post by rjm » January 2nd, 2009, 7:00 pm

Gussets on butt joints should be simple, rectangular pads glued over the joint so that the grain of the gusset crosses the joint. If you put the grain of the gusset parallel to the joint, it will split between the fibers of wood in the gusset. Think of it as similar to gluing threads across the joint to hold it together, except that the threads are really wood fibers. The load on such a gusset is mainly tension. It will see some shear loading as the bridge twists or sags. I'd suggest making the gussets as thin as possible.

There's an interesting point to be made about the wood gussets in the photo. They are installed to add stiffness to their joints to resist bending. These joints are part of a rigid-frame structural bent made by mitering 2x6 rafters and laminating plywood to each side. These gussets not only hold the joint together, they preserve the angle of the rafter. As the joint is loaded, it tends to open up like scissors handles (it does the splits - love that expression), so the gusset is mainly loaded in shear. That's usually not necessary in SO bridge joints, so a simple, thin gusset which just spans the joint should be enough. Triangulation of the bridge structure should preserve the joint angle

I'd still recommend lap joints for most purposes.

Bob Monetza
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Re: Elevated Bridge 2009

Post by croman74 » January 2nd, 2009, 9:09 pm

For the pieces that connect one top chord to the other, should you have them on the outside like this one
10/IMG_0080.jpg

or on the inside like this one
10/IMG_6722.jpg
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Re: Elevated Bridge 2009

Post by Keyreaper » January 2nd, 2009, 11:33 pm

So far, the highest efficiency I've seen was a 10 gram bridge holding all 15 kilos...(for those of you that can't do mental math, that equals efficiency of 1500) It was built by St.Joesph in MO I believe. The one trailing after that was a bridge that was around 14-16 grams, if i recall right, holding between 10 and 11 kilos. (about an efficiency 700) Then our team placed 3rd with a 14 gram bridge holding around 8 kilos. (about 500) However. There were at least 10-15 more people between third and second that were from the same school that placed in second, and their bridges didn't count because only one school could only have one bridge place, meaning the highest one could place. But then you guys probably know that :D . The efficiencies of most bridges I've seen range usually from 200-500. But there have been a couple over 500, and under 200. Mine placed around 100 :lol: EPIC FAIL! But the bridge that I am now building will do a lot better, because I threw together the first bridge in about 4 hours...hehe

Anyways, at the next competition I will ask for a sheet of the efficiencies of all bridges that were tested, and hopefully post it on here.
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