There is this West Point Bridge Design Competition every year. You get a program, available on line
http://bridgecontest.usma.edu/index.htm, and build a simulation of a truss bridge. I have done this for several years, and I found the best truss design was the equalateral triangle. It evenly distributes the forces and holds up under strange cercumstances. If you use the right triangle the perpendicular edge takes a lot of compression and fails much sooner. Download the program and test some different ideas. It's a fun contest.