For its weight, balsa wood is one of the strongest building materials available. It's a perfect material for a number of small-scale projects, including making model bridges. The bridge itself is something of a marvel as well: spanning a gap and holding up not only its own weight, but the weight of passing vehicles. A strong model bridge begins with a good design and stops with solid construction practices before ending with preparation time.
Measure the gap your bridge will span. Decide on a scale for the grid paper, such as one square representing 1/2-inch. Draw fixed points that represent the edges of the gap about five squares from the bottom. Draw them the correct number of squares apart. Extend lines down and away from the gap to create L-shapes.
Diagram the bridge. Start by drawing a line across the gap representing the bottom, then begin to add support structures. Consider existing bridge designs while designing. Find a number of successful designs on the web. Bear in mind also that the triangle is one of nature's strongest shapes.
Plot the forces acting on the bridge. Pencil in an arrow from the center of the bridge to represent the weight that will be pulling on it. Draw smaller arrows pulling or pushing on certain parts of the bridge as you imagine how that force will be distributed. Add more supports if you feel they are necessary.
Cut out balsa wood pieces to represent each of the pieces on your diagram. Use 1/8-inch balsa wood to make up the bottom of the bridge and the exterior of the support system. Make the central support struts and the base of the bridge if you have some wood left over. The 1/16-inch balsa wood works well for trusses and other support beams. Cut the pieces to scale and make clean cuts.
Assemble the bridge. Glue the 1/8-inch pieces together, then slide the trusses and support beams into place. This is where the clean cuts prove essential, as they will create more solid connections to the rest of the wood. Leave the bridge to dry overnight.
Test the bridge by applying some weight to it. Remember that not all bridge designs will be successful. You can learn a lot if your design does fail. Examine the damage to the bridge to figure out which areas need additional support and what other design improvements you can make.