Have to build a balsa wood bridge for school.. help!?

From experience i know that the best way to build a bridge with balsa wood is not to build high. Although it may be a more beautiful design, it tends to become unstable and colapse real fast. Make a kind of rigid beam out of 4 single beams which are connected with n- or z-shaped trusts. Once it's nearly done always concentrate on the weakest link and reinforce it! Best of luck and have fun!

Build a truss that looks like a bunch of letter "N"s touching each other with the slanted piece pointing down and towards the center of the span. Observe this design on overhead traffic signs on the Interstate highways around town. These trusses stretch almost 100 feet and aren't very big otherwise. Then put two trusses parallel and connect thim with cross members going straight from side to side.

There's not really a formula (aside from sum of forces = 0 and sum of moments = 0), just more of a methodology. Try to get your hands on a engineering statics book and look up simple trusses.

How big is it supposed to be? To build your trust the easiest thing would be to put 1 column in the middle and 2 diagonal columns from the top of the that to the corner of the truss, if you like you can email me more info and I could put something together for you.

My email is ralph.leyva@sbcglobal.net

If your pieces are say, 1/2" x1/8", attach the 1/2" side vertically for strength. since the forces on the bridge are downward i.e. gravity and the load on the bridge - such as "miniture" cars

Look on the net I'm sure there would be a lot of suggestions