What some specs on what you are trying to do?

As in, height, width, length, type of wood, etc that you need to use.

Also, how much weight does your bridge need to hold?

Okay, first take a 5 gallon bucket and fill it with water and weigh it. Now you know, in pounds, what the bridge has to hold. Since you can use pretty much anything, the first thing that comes to my mind is metal. However, I don't know where you could get the pieces you need, and you would have to have some soldering skills.

You could try using wood. Because you can only use pieces no longer than 6 inches, you are going to have a lot of joints.

Are there any other constraints? How long does your bridge have to be?