When building a balsa bridge or tower, you need to pay attention to the quality of the wood, the properties of the glue, and whether your pieces are straight and level.
Use the Right Type of Balsa Wood
Balsa wood varies significantly in weight, ranging from 4 pounds per cubic foot to 24 pounds per cubic foot. If your structure has a weight limit, you'll need to select your balsa carefully. Heavier balsa is stronger, and should be used for areas of the bridge that will be subjected to the stress of compression, while lighter pieces can be used for connections that are not weight bearing. Sometimes balsa wood has insect holes or air pockets that can create areas of structural instability. Sometimes these are internal, and can't be seen, but carefully check each piece externally anyway to identify potential vulnerable locations.
Select Glue Carefully
Glue varies widely in how fast it sets, whether it fills gaps well, and how strong a joint it creates. If you are building in class and need to test the same day, a cyanoacrylate, or CA, glue would be a good choice. CA glues are also easy to apply in small amounts and are light weight. If weight is not an issue, epoxy will produce a joint that is stronger than the wood surrounding it. Wood glues tend to penetrate the wood itself, providing additional structural stability, but the joints need to be clamped and won't be totally set for 24 hours.
Use a Jig to Build a Straight and Level Structure
The best balsa bridge design will fail if the pieces are not cut straight and level. This is because a lopsided piece of wood will not distribute the forces evenly, causing torque which will increase the stress on the joints and members. Using a tabletop miter saw made for hobbyists will help ensure your cuts are straight, and assembling your bridge on a jig will help make sure it's level. A simple jig for each side of the bridge can be made by laminating graph paper and attaching it a piece of pink foam insulation. Use a ruler to carefully mark the design of your bridge on the graph paper, and lay out the members for each side of the bridge on the outline.