Truss bridges support the weight of a bridge by spanning a series of trusses from one end to the other, supporting the weight of the bridge beneath. There are several types of truss bridge designs. The Warren truss uses equilateral triangles. The Pratt and Howe truss also uses triangles, but the triangles are slanted to change the distribution of weight. The K truss uses the same shape as the letter "K" to distribute weight. A model truss bridge can be made from balsa wood using any of these designs, or you can construct a truss bridge of your own design.
Gather pictures or designs of the truss bridge you would like to construct, or use them as the basis for your own design. Pay close attention to the details of the design, including how the trusses are connected to the bridge, where the joints connect, and how the design of the structure distributes loads on the bridge to the trusses.
Determine the scale you would like to use. For example, a 1:12 scale would mean that every foot of a life-size bridge would be one inch on your model.
Draw the design of your bridge on a piece of graph paper showing the side view of the bridge. Use the squares on the graph paper to correspond to the units of your model. For example, one square might equal one inch on your model.
Draw a second view of your design, looking down from the top, on a second piece of graph paper.
Determine the amount of balsa wood required for your bridge by measuring your designs with a ruler.
Measure two pieces of 1/8-inch balsa wood for the base of the bridge. Cut your pieces of balsa wood using a ruler and a sharp utility knife. Score the wood with moderate pressure several times. Do not try to cut balsa wood in a single cut, as too much pressure will crush the edges of the wood.
Measure and cut the pieces necessary for your trusses, using 1/16-inch balsa wood, according to your design. Use whole pieces of wood wherever possible. Where joints between pieces are made, cut the wood so weight will be put on wood, rather than on glue. For example, a vertical beam would be placed on top of a horizontal beam, not glued to the side.
Glue the pieces together using white glue. Clamp larger pieces with wood clamps where possible. Use masking tape to hold together smaller pieces until the glue has dried.
Test the strength of your bridge by placing a small amount of weight on the bridge, then gradually increase it until you are satisfied your truss bridge is of sound design and construction.