you can also use balsa wood to build a model Truss bridge at home

Bridges are complex structures that are marvels of science and engineering. They have forces to counteract, including standing up under their own weight. The Truss bridge design is a common bridge style that makes use of triangular trusses to hold the base of the bridge together. Although students make models of Truss bridges out of surprisingly light and strong Balsa wood in physics classes across the country, you can also use balsa wood to build a model Truss bridge at home.
Diagram the bridge on your grid paper, using the grid squares as a guide. To make a scale model of a bridge, using grid-square to represent 0 square-feet at actual size, and square-inch at model size will give you a good scale for the design. A truss bridge usually has the flat base of the bridge and a trapezoidal arch across the top, to which the trusses are connected. Draw the trusses in as well. Make several copies of the diagram, and draw it from a few angles--like the side, top and /-rotational view.
Using a side diagram, imagine how the forces will act on the bridge as something crosses it. Some of your trusses will be pulled and others will be squeezed, so make sure these parts are prepared for that kind of force. Draw arrows at the nodes (points where trusses meet the bridge base) and imagine how those arrows will pull all parts of the bridge. Make any changes necessary based on this diagram. See the "building bridges" resource for a clearer, more scientific picture of how these forces will act.
Measure and mark the two base pieces of the bridge, and the arched top pieces. Ideally, these should all be single, continuous pieces of balsa wood--if not, make extra pieces to splint together. Cut these pieces out, taking care not to mash the wood while cutting--damaging the fibers can weaken the wood and weaken your bridge.
Measure and cut the arch support beams and a series of pieces to make up the bridge base. Miter them if possible, as a cleaner angle will hold together better.
Glue the base together, and glue the arches together. Hold these pieces tightly together, using masking tape where possible to reinforce the joint while the glue dries.
Measure, mark and cut the outermost set of trusses. There should be four pieces. Glue them in place, and hold until the glue dries.
Repeat step with the next set of trusses, then the next, and so on until you reach the middle.
Leave the bridge to dry overnight. Test the bridge with a light load to see how well it holds up. Make any repairs as necessary. For more you can visit our webiste: http://www.balsafactory.com/.