Building a bridge

Quin

Quin

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Dec 5, 2006
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In one of my classes, we're building a bridge out of two 90cm lengths of basalwood, and two 60cm lengths of basswood. We're allowed 300cm of 8lbs test line, 50 toothpick-ish objects (little longer, square on the ends, little thicker) and 20 craft sticks (pretty thin but stack easily. I estimate length at 10cm). Any adhesive may be used, but no other materials other than what is provided. The wood may be filed, ground down, etc. It must be 30cm long and support a small platform that is approximately 2cm thick, 10cm long and 4cm wide. A small testing machine that continually and gradually pulls down, adding more and more force will be attached to the platform.

My plan was to use the real wood to triple layer the side pieces, and have the odd piece out cut into small pieces. Construct a cocoon of sorts out of the fishing line, but cut in half in the middle. JB Weld all over the outside to decrease chance of stretching (open to suggestions on better material to use, but remember it must be an adhesive), and shove all the little sticks in the cocoon as additional support to prevent the fishing line from giving in.

Completely terrible idea? Better suggestions?
 
Facime

Facime

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Jun 1, 2006
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I would use wood glue, its as strong or stronger than the wood itself when used properly. I agree to look to real world examples and build off that. I think if I were doing it I would first look at a modified arch support. You want to have the pressure be transfered into the length of the span rather than the width. Good luck.
 
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supramk3man

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Jun 26, 2005
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This is some what cheating but in tec class, I like most of you all had this same task. I put stick pins in the joints, worked great till it got broke and you could see the pins. But it held alot of weight. Just a way to show off if you don't get caught.
 
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foreverpsycotic

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Jul 16, 2006
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Can it touch the area underneath the bridge at all? If not, 2 supports dirrectly in the middle triangulated to the sides

What are the dimentions of the bass wood, and the balsal?

God, this reminds me of physics in HS, where I made the strongest index card tower, held some absurd amount of weight and I used the least amount of cards in the class.
 

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Quin

Quin

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Dec 5, 2006
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Fishing line suspension bridge was my first thought too, but it can't hang below the structure. Rules suck. Arch was my next thought, but this wood really can't be bothered into making an arch. Flimsy as hell. I snapped two sticks trying to do this. I'm hesitant to look at modern bridges for inspiration, as concrete and rebar are completely different materials than wood. foreverpsycotic, with your plan the platform would be suspended below the structure you posted a rough sketch of, right? So that was basically a top view?


This a truss, right? Excuse my shitty paint skills.
 

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foreverpsycotic

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side view. if you take the bottom out, it should still hold up just fine. the trick to the flex would be to use the balsal wood as only a top cover over the bass, using the bass as runners along the side, and for the pillars. Balsal sucks structurally. Off the wall idea, but you might want to look into drilling small holes in the runners, to allow for more flexability in the structure, and less of just a snap. Once you get everything set up in a box shape, use the fishing line and go from hole to hole, like you are lacing shoes. Check these views out. BTW, the bottom one is top, with support lines between the pillars.
 

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Keros

Keros

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Mar 16, 2007
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I would build a beam and platform system, with the above road level support system foreverpsycotic proposed.

Make the beams as deep as possible, using an I beam as an example. The deeper the beam and wider the flange, the stronger it will be. I don't know how much material you have to do it, but 3 beams (left, right, and middle), and then a few cross braces in an X formation down the diaphragm of the beam, would be extremely strong.

If you're real hardcore, look up prestressing. If you can prestress the bottom chord of a wood beam, placing it into an arched shape, it will support a rediculous amount of weight. Alot more weight than the beam could ever support otherwise.

Wood, concrete, rebar, plastic, the design principles are the same.
 
Quin

Quin

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It's not a bridge in the traditional sense. It holds that little platform thing and weight is added to the bottom of it until it snaps
 
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