It is very important to remember that different construction materials have different useful properties. Metals have strength when they are pulled (tensile strength), wood and concrete have poor tensile strength but very strong compressive strengths. Before you begin building your bulsa (a compression resistant wood) bridge, take a drive around your area and look at various bridges. Sketch what the metal ones look like, and what the concrete or wooden ones look like. Learn lessons from past engineers!     You now see that it is not a good idea to build a wooden trestle bridge as most students will want to do. Theirs will look so strong but they are not made of steel! Read your rules very carefully, and try to build a span and supporting pile bridge if allowed. Or maybe an "overshot" bridge (roadway on top) with diagonal supports underneath (compression!). Remember that a square inch of upright bulsa will hold more than 300 pounds without even a creak! For more details: www.science-projects.com/BulsaBridge.htm