- Rules normally dictate that the materials used must be commercially available rectangular balsa wood stock and glue. You can buy this wood from any hobby store or even most hardware supply stores. Just remember when selecting it, not all balsa wood is created equal. Since you'll be selecting this wood for a competition, make sure to avoid pieces with pitting and warping. Not only will these impede the building of your bridge, they may very well make it weaker.
Also, remember that the rules tend to dictate that you can't use any balsa wood besides these long, thin rectangular pieces. But that doesn't stop you from gluing them together side by side to create thicker pieces. - Designing the bridge is the most difficult and painstaking part of the whole process. Certain shapes are better at holding certain levels of load stresses without breaking. Generally speaking, triangles tend to be stronger as they disperse stress evenly in multiple directions. The typical bridge design that utilizes triangles the best is a truss bridge, on which supporting trusses are built along the side of the bridge. These supportive trusses incorporate a series of triangles to ensure that weight on the bridge is dispersed across the entire design.
- The joints and intersections of your bridge are going to be the weakest points on your bridge. As such, it is important that you pay extra attention to reinforce these and make sure that the wood fits as flush together as possible. Do not be afraid to take after these ends with a sharp hobby knife to make sure they fit together perfectly.
It is also imperative that you resist the urge to over-glue everything. You are operating under a strict weight limit, and glue is heavy. If you use a properly designed bridge, you need only to make sure the glue holds the wood pieces in place. Let the wood worry about bearing the load.
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