Building a popsicle stick bridge
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Posted by: Schmerdro

02/20/2006, 07:02:43

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I have to talk to a civil engineer about how I can quantify when my bridge will break. My teacher said he knows somebody and 2 other students said the same thing but so far no one has received a response from them.

So, this is what my bridge is going to look like:
http://img157.imageshack.us/img157/3889/printscr6cb.png

That's the side view. I would like to know if I hang a mass from the middle of the bridge, how is the force going to distribute itself if all of those triangles are equilateral? This mass will be increasing systematically (at a constant rate).
So, basically, I need my dependent variable that will change with my independent variable (which is the mass hanged from the bridge).

The bridge will be 57 cm long at the bottom and 45.5 cm long at the top. It will be placed on 2 desks that are 40 cm apart and I will be using hot glue.

Since a bridge is supposed to distribute its load towards its extremities, I was thinking of placing the bridge on 2 mass weighters... and then the more those mass weighters will indicate (compared to the load that is actually hanged from the bridge), the more efficient the bridge is. Is that mathematically correct?

Also, can you tell by just looking at my design where it will break? I'm thinking it will break here (in the red sticks, on the black area):
http://img79.imageshack.us/img79/2381/printscr6cb8dn.png
Because my top and bottom will be heavily reinforced and those 2 sticks will only be reinforced by 1 popsicle stick on both sides and they will be compressed. I'm not worried about tension because the hot glue is incredily strong and tcisk are more likely to break under compression than tension.








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Re: Building a popsicle stick bridge
Re: Building a popsicle stick bridge -- Schmerdro Post Reply Top of thread Forum
Posted by: swearingen

02/20/2006, 07:18:40

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Just by looking at it, I'd say the chords will break at the ends in bending. You probably won't understand what that means, but to help you out, let me direct you to a great program. Go to http://bridgecontest.usma.edu/ and download the software. This is a contest put on by West Point Military Academy every year. Playing with their software for 30 minutes will teach you more about bridges and trusses than me banging out text for a month. Go ahead and start from scratch and build your bridge as you have it drawn. You'll see the problems immediately and if you poke around a little, you'll be able to fix it.

PS: Popsicle sticks are MUCH stronger than hot glue. Your bridge will probably fail at the joints. The trick is to keep tension loads (which can only be taken by the glue) less than compression loads (which can be taken by the much stronger sticks, if arranged correcly). Also, the red members you show on your second picture will not fail first.

Good luck!!








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Re: Building a popsicle stick bridge
Re: Re: Building a popsicle stick bridge -- swearingen Post Reply Top of thread Forum
Posted by: Schmerdro

02/20/2006, 07:33:00

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Yes, I already have used that program.

In the following picture, the red side will be supported by the object on which the bridge will be placed:
http://img.photobucket.com/albums/v719/Lacadaemon/Bridge.gif
So those chords at the end are pretty much meaningless.

"the red members you show on your second picture will not fail first"
Then, assuming the bridge will not break at its joints, which members will break first?







Modified by Schmerdro at Mon, Feb 20, 2006, 07:34:17


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Re: Building a popsicle stick bridge
Re: Re: Building a popsicle stick bridge -- Schmerdro Post Reply Top of thread Forum
Posted by: swearingen

02/20/2006, 13:42:14

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You can understand why I was concerned about those end diagonals if you've used the program. Your picture cleared it up.

Now, if you go back and put your bridge (without the end diagonals) into the program, you'll actually see which members would fail first if the joints don't. If you set it up where all of the members are the same size, material, and cross section, it doesn't matter what the material is, the same member will fail in the model as in the program. You must watch to see which member gets the darkest red or blue when the rear tire of the truck gets to the middle of the bridge because that's where the weight will be placed in your competition. See if you can answer these questions before you model it: will the members get darker red or blue, i.e. will there be a compression or tension failure? Which members will get the darkest? Then see if you're right.

By the way, I assume that the black lines lapped over the white are popsicle sticks lapped over each other.








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Re: Building a popsicle stick bridge
Re: Re: Building a popsicle stick bridge -- swearingen Post Reply Top of thread Forum
Posted by: ChrisMEngr

02/20/2006, 21:27:59

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Do you have to use hot glue?

I am not sure if you are trying to make it stronger but if you are, Elmers wood glue should be much stronger than hot glue.








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