>_< I build a balsa wood bridge MANY years ago. Use X's and Triangles is the most I can say. I used one of those under trussel things to distribute the weight. There is a west point bridge builder simulator, use that.
I'm not sure what type of pasta I should use though. I've tried penne, but it seems like it'll add too much weight. I had to stuff the insides with more penne to make it more rigid and stable, but the flipside is that it is alot more heavier. I do have a design, just not sure how to achieve it with pasta. -_-