# Toothpick bridge design

1. Sep 14, 2009

### technoweasel

I have a question about bridges. I am not sure if this is the right place, since everyone else is discussing more advanced things, but I couldn't find anything (I did try to search). If this post is out of place just please redirect me.

For school I need to design and then build a bridge to connect 2 supports that are about a foot apart, using only toothpicks and wood glue. My question is this: Would designs for real bridges work in this situation, even though the force will only be applied to a point in the center of the structure? I have read a bit about the parabola and catenary in calculus books, but would a curve be best in this situation? If a cable supporting a uniform weight (along the x axis) makes a parabola, and it can be inverted to create an arch with the forces reversed, then can a cable supporting a single weight that pulls it down in a V shape be reversed to make an optimal arch for this application?

What is wrong with my reasoning? I haven't been through physics in school yet, so I might not even be asking the right questions. Please help me by explaining what is going on or redirecting me to some good online resource.

2. Sep 14, 2009

### nvn

Yes, real bridges would work in this situation. An arch would perhaps be optimal, but an inverted V would work well, also. Your reasoning sounds correct.

3. Sep 15, 2009

### archis

The ''best'' curve depends on that how force is applied to the bridge. The bridge form should mirror bending moment diagram. For point force in middle it would be shape of a pyramid, for distributed force it is an arch.

4. Sep 15, 2009

### nvn

Makes sense. I currently agree with the post by archis now, which matches the concept posted by technoweasel. And by the way, technoweasel, it doesn't really matter if the V is upright (below the bridge) or inverted.

5. Sep 16, 2009

### technoweasel

You're right, nvn.

Thanks, guys. If I wasn't clear, the "bridges" do not require a road deck, and the instructor adds weights to an area near the center of the span until the structure collapses. Are there any web pages that discuss this in detail? I might use a curve anyway because I have to do a report and it would be much more interesting to write about curves and their derivation. However, I guess I could write about the caterary and parabola and why I decided NOT to use them. I will continue researching.